UNIVERSITY  OF  CALIFORNIA 

ANDREW 

SMITH 

HALLIDIL: 


v  STANDARD 
POLYPHASE  APPARATUS 

AND 

SYSTEMS- 

BY 

MAURICE    A.   OUDIN,   M.  S. 

Mem.  Am.  Ins.  E.  E. 


WITH  MANY  PHOTO-REPRODUCTIONS, 
DIA  GRA  MS  A  ND  TA  RLES 


Third  Edition,  Revised 

NEW    YORK: 

D.  VAN     NOSTRAND    COMPANY 

23  MURRAY  AND  27  WARREN  STREETS 


LONDON: 

SAMPSON    LOW,    MARSTON    &    COMPANY 

LIMITED 

St.  3Dunstan's  Spouse 

FETTER  LANE;  FLEET  STREET,  E.G. 
1902 


HM.UD1E 


COPYRIGHT,  1899,  1902 

BY 
D.  VAN  NOSTRAND  COMPANY 


C.     J.     PETERS     &     SON,     TYPOGRAPHERS, 
BOSTON. 


PREFACE    TO    THE    THIRD    EDITION. 


THE  publication  of  a  third  edition  of  this  little  work  has 
enabled  the  writer  to  bring  it  up  to  date.  While  the 
applications  of  polyphase  working  have  extended  enor- 
mously since  the  first  appearance  of  the  book,  there  has 
been  but  little  deviation  from  the  methods  laid  down 
therein  as  standard. 

The  three-phase  system  has  been  found  to  be  almost 
universally  applicable.  It  has  largely  replaced  the  two- 
phase  and  to  a  still  greater  extent  the  monocyclic  system. 

Marked  features  of  the  recent  development  of  electri- 
cal manufactures  are  the  increased  size  of  generators  and 
of  power  consuming  machinery,  and  the  creation  of  new 
devices  for  their  control.  The  most  progressive  engineer- 
ing work  of  the  day  is  that  of  switchboard  design. 

JANUARY,  1902. 


CONTENTS. 


CHAPTER  PAGE 

I.    DEFINITIONS  OF  ALTERNATING-CURRENT  TERMS  .    .  i 

II.     GENERATORS     ................  17 

III.  GENERATORS  (Concluded}     ...........  37 

IV.  INDUCTION   MOTORS     „     . .     .    .  63 

V.    SYNCHRONOUS   MOTORS    .    .    ..........  94 

VI.     ROTARY   CONVERTERS  .    .    ', .109 

VII.     STATIC   TRANSFORMERS    . 125 

VIII.    STATION   EQUIPMENT  AND  GENERAL  APPARATUS  .    .  145 

IX.     TWO-PHASE  SYSTEM 180 

X.     THREE-PHASE  SYSTEM 194 

XI.     MONOCYCLIC  SYSTEM 212 

XII.     CHOICE  OF  FREQUENCY 224 

XIII.  RELATIVE  WEIGHTS  OF  COPPER  FOR  VARIOUS  SYSTEMS,    231 

XIV.  CALCULATION  OF  TRANSMISSION  LINES 238 

Appendix  —  THE  STANDARDIZATION  OF  GENERATORS, 

MOTORS  AND  TRANSFORMERS 261 


STANDARD   POLYPHASE  APPARATUS 
AND  SYSTEMS. 


CHAPTER    I. 
INTRODUCTORY. 

DEFINITIONS    OF   ALTERNATING-CURRENT 
TERMS. 

Alternating  Currents.  —  On  account  of  the  limitation 
imposed  by  the  space  of  this  book,  mathematical  demon- 
strations of  alternating-current  phenomena  have  been 
omitted  in  the  following  pages,  and  the  chapter  will  be 
found  to  consist  mainly  of  elementary  explanations  •  and 
statements  which  partake  of  the  nature  of  definitions.  It 
is  hoped  that  these  definitions  will  be  found  useful  in 
aiding  the  uninformed  reader  to  obtain  a  clearer  under- 
standing of  the  principles  underlying  polyphase  appa- 
ratus and  methods.  For  a  more  comprehensive  treatment 
of  alternating-current  phenomena,  the  reader  is  referred  to 
the  many  works  on  the  subject. 

The  alternating-current  generator  was  one  of  the  ear- 
liest applications  of  the  principles  of  induction.  Unlike 
the  current  from  the  direct-current  generator,  which  came 
at  a  later  date,  the  alternating  current  rapidly  changes  its 
value  and  direction,  the  fluctuations  being  periodical. 
Such  a  current  reaches  a  maximum  in  one  sense,  de- 

i 


2  POLYPHASE   APPARATUS   AND   SYSTEMS. 

clines  to  zero,  reverses,  and  then  attains  a  maximum  in 
the  other  sense,  as  often  as  the  pressure  of  the  generator 
follows  this  variation.  This  variation  of  current,  or  of 
pressure  in  its  simplest  and  ideal  form,  follows  the  law 
of  simple  harmonic  motion,  and  may  be  represented  by 
the  projection  of  a  point  moving  in  a  circle,  with  a  con- 
stant velocity,  upon  a  perpendicular  diameter. 

The  development  of  this  motion  and  its  application  to 
the  variation  of  the  current  or  the  induced  pressure  of  an 
ideal  alternating-current  generator  is  illustrated  in  Fig.  i. 
The  point  P  on  the  circle  is  considered  as  moving  with  a 


270' 


Fig.    1. 


constant  velocity.  Its  projection  on  the  diameter  is  the 
value  of  the  pressure  at  any  instant  of  time.  The  circle 
represents  a  complete  revolution  or  cycle  of  change  of 
current  or  pressure.  The  straight  line  to  the  right  is  the 
development  of  the  circle  expressed  in  degrees,  360  of 
which  constitute  one  complete  period.  On  this  line  the 
instantaneous  values  of  the  current  or  the  pressure  derived 
from  the  projection  of  P  are  plotted.  It  is  seen  that  a 
line  drawn  through  these  points,  obtained  for  the  com- 
plete revolution,  gives  a  sine-curve. 

On  account  of  the  irregular  magnetic  field,  in  practice 
few  alternating-current  generators  give  rise   to  pressures 


ALTERNATING-CURRENT   TERMS.  3 

following  a  simple  sine-law.  The  departure  from  a  sine- 
curve  is  not  so  great,  in  the  majority  of  alternating-current 
generators,  but  that,  for  purposes  of  most  commercial  cal- 
culations, their  electro-motive  forces  can  be  considered  as 
simple  harmonic  quantities. 

The  formula  for  the  flow  of  current  in  an  alternating 
system  of  conductors  is,  in  its  general  form,  similar  to 
that  used  for  determining  the  flow  in  a  direct-current 
system.  It  differs  from  Ohm's  law  only  in  the  introduc- 
tion of  certain  factors,  which,  however,  may  become  so 
complex  as  to  conceal  the  simple  quantities  of  the  equa- 
tion, resistance  and  E.M.F.  The  value  of  these  factors 
depends  on  three  well-known  properties  of  a  conductor. 
These  are : 

1.  Inductance. 

2.  Capacity. 

3.  Virtual  Resistance. 

Inductance.  —  The  magnetic  field,  surrounding  a  circuit 
through  which  a  current  is  flowing,  exerts  no  influence  on 
the  circuit  in  the  case  of  a  direct  current  of  constant  value. 
In  the  case  of  an  alternating  current  it  is  of  far  greater 
practical  importance,  and  gives  rise  to  a  variety  of  phe- 
nomena. The  magnetic  flux  then  varies  periodically  with, 
and  in  the  same  manner  as,  the  current  and  E.M.F.  The 
setting  up  of  this  magnetic  flux  —  or  lines  of  force,  as  they 
are  sometimes  called  —  produces  an  E.M.F.  in  the  circuit, 
in  opposition  to  the  induced  E.M.F.  This  counter  E.M.F., 
or  E.M.F.  of  self-induction,  is  stronger  when  the  magnetic 
flux  is  changing  most  rapidly  ;  therefore  arriving  at  a  max- 
imum, 90°,  later  than  the  flux  and  the  current  producing 
the  flux.  The  result  of  this  counter  E.M.F.  is  that,  when 


4      POLYPHASE  APPARATUS  AND  SYSTEMS. 

an  external  E.M.F.  is  applied,  the  current  does  not 
immediately  attain  its  maximum,  and,  when  the  E.M.F.  is 
withdrawn,  the  current  persists  for  awhile.  The  current 
reaches  its  maximum  later  in  point  of  time  than  the 
E.M.F.,  —  i.e.,  is  always  lagging  behind  the  E.M.F.  It 
would  seem  as  if  a  current  of  electricity  possessed  a 
quality  of  the  nature  of  the  inertia  of  matter. 

The  strength  of  this  flux,  or  the  induction  as  Faraday 
called  it,  is  determined  by  the  current.  The  extent  to 
which  a  gixen  flux  affects  a  circuit  in  a  non-magnetic 
medium  —  i.e.,  the  magnitude  of  the  counter  E.M.F.  — 
depends  solely  upon  the  geometry  of  the  circuit.  If  the 
circuit  is  wound  in  a  coil,  or  so  arranged  that  in  the 
periodic  variation  of  the  flux  the  same  lines  of  force 
encircle  more  than  one  portion  of  the  conductor,  the  coun- 
ter E.M.F.  will  be  increased. 

That  constant  quality  of  a  circuit  which  determines  its 
inductive  effects  is  called  inductance.  The  inductance 
may  be  either  self  or  mutual  inductance,  according  as  the 
circuit  is  isolated  or  acted  on  by  an  adjacent  circuit,  also 
carrying  a  current.  Inductance  is  frequently  called  the 
co-efficient  of  induction.  The  symbol  L  is  used  to  desig- 
nate self-inductance,  —  the  unit  of  measurement  of  which 
is  the  henry. 

Capacity.  —  Like  inductance,  the  capacity  of  a  circuit 
depends  upon  its  geometry  and  its  surroundings.  It  is 
the  quality  which  a  conductor  possesses  of  being  able  to 
hold  a  quantity  of  electricity.  A  combination  of  con- 
ductors or  conducting  surfaces,  advantageously  placed  to 
hold  the  greatest  possible  quantity  of  electricity,  is  called 
a  condenser.  All  insulated  lines  act  more  or  less  like 
condensers.  The  charging  or  discharging  current  of  a 


ALTERNATING-CURRENT   TERMS.  5 

condenser  is  greatest  when  the  rate  of  change  of  effective 
pressure  is  greatest ;  that  is,  when  the  E.M.F.  is  at  zero  at 
the  moment  of  passing  from  negative  to  positive,  or  vice 
versa.  The  effect  of  capacity,  then,  is  opposite  to  the 
effect  of  inductance,  and  may  neutralize  it,  or  even  over- 
come it,  when  existing  in  the  same  circuit.  In  a  circuit 
having  capacity,  the  current  may  ]ead  the  E.M.F.  in  phase. 
Fig.  2  shows  the  lead  produced  by  capacity.  The  curve 
V  represents  the  curve  of  E.M.F.,  and  /  the  current 
curve  leading  the  E.M.F.  The  unit  of  measurement  of 
capacity  is  the  farad,  and  is  usually  represented  by  the 
symbol  K. 

Impressed  E.M.F.  —  The  more  frequently  an  alternat- 
ing current  is  re- 
versed, the  less 
time  is  there  avail- 
able for  it  to  reach 
the  value  it  would 
have  in  a  direct- 
current  system. 

-T-       i    •         ^i   •  •  Fig.    2. 

To  drive  this  maxi- 
mum current  through  an  alternating  system  of  conductors 
havinginductance, requires  a  greater  E.M.F.  than  is  needed 
in  a  direct-current  system  to  produce  this  same  current. 
The  inductance  of  a  circuit,  as  explained,  determines  the 
counter  E.M.F.  ;  and  it  must  be  overcome  by  an  added 
amount  to  the  E.M.F.  required  to  produce  the  same  cur- 
rent in  a  direct-current  system.  The  name  of  impressed 
E.M.F.  has  been  given  to  this  resultant.  The  counter 
E.M.F.  acts  at  right  angles  to  the  current,  and  is  greatest 
when  the  current  is  reversing  its  sign,  or  when  the  rate  of 
change  of  the  lines  of  force  is  greatest.  The  values  and 


6      POLYPHASE  APPARATUS  AND  SYSTEMS. 

direction  of  the  impressed  E.M.F.  and  its  components  may 
be  considered  in  a  diagram.  In  Fig.  3  the  impressed 
E.M.F.  is  shown  as  the  hypotenuse  of  a  right-angled  tri- 
angle. That  component  of  the  E.M.F.  which  would  drive 
the  same  current  through  a  circuit  without  inductance, 
being  necessarily  in  phase  with  the  current,  is  shown  as 
lagging  behind  the  impressed  E.M.F.  by  an  angle,  3>,  and 
by  a  length  equal  to  its  magnitude.  In  quadrature  with 
the  component  is  the  E.M.F.  of  self-induction,  the  mag- 
nitude of  which  determines  the  length  of  the  line  in  the  dia- 
gram. The  magnitude  of  the 
impressed  E.M.F.  is  then  rea- 
dily  found.  The  name  of  en- 
ergy  E.M.F.  has  been  given 
to  that  component  in  phase 
with  the  current,  and  which  is 
effective  in  doing  any  work  in 


IR  Energy  EMF  .          .  .          . .      . 

a  circuit.     As  all  the  quanti- 
ties in  the  diagram  must  follow 

the  law  of  simple  harmonic  motion,  the  curve  of  self-induc- 
tive E.M.F.  will  be  shown  in  the  same  way  as  the  curve  of 
impressed  E.M.F.  The  effect  of  this  inductive  component, 
in  increasing  the  impressed  volts  needed  to  cause  a  given 
current  to  flow,  is  shown  in  Fig.  4.  The  curve  RI  repre- 
sents the  energy  component  of  the  impressed  E.M.F. 
which  would  drive  the  current  if  there  were  no  induc- 
tance. It  is  equal  in  value  to  the  product  of  the  current 
and  the  resistance.  In  quadrature  with  it,  is  the  inductive 
E.M.F.,  designated  by  the  curve  /Z7,  /  being  equal  to 
2irN9  where  N  is  the  number  of  complete  cycles  per  second, 
and  L  the  inductance.  This  is  the  component  required  to 
offset  the  effect  of  the  inductance.  By  adding  the  ordi- 


ALTERNATING-CURRENT   TERMS.  7 

nates  of  the  two  curves, -we  obtain  a  third  curve,   V,  also 
following  the  sine-curve  law.     This  is  the  curve   of  the 


Fig.  4. 

impressed  E.M.F.  required  to  produce  the  given  current 
in  this  particular  circuit. 

Impedance  ;  Reactance. --(;  Impedance  is  the  total  opposi- 
tion in  a  circuit  to  the  "flow  of  current.  \  It  determines  the 
maximum  current  that  can 
flow  with  a  given  impressed 
E.M.F.  It  is  made  up  of 
a  resistance  component  and 
another  component  to  which 
the  name  of  reactance  has 
been  given.  The  relations 
of  resistance,  reactance,  and 
impedance  are  shown  in  Fig. 
5.  As  there  may  be  energy 
losses  external  to  a  circuit,  and  yet  dependent  on  that  cir- 
cuit, which  require  a  flow  of  current  that  cannot  be  deter- 
mined by  a  calculation  based  upon  the  ohmic  resistance 
alone,  it  is  not  correct  to  designate  the  resistance  com- 
ponent as  the  ohmic  resistance.  Such  losses  are  those 


R-Resistance 
Fig.   5. 


8 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


R  -Resistance 


82 
II* 

I5H° 
ir 

o 


due  to  hysteresis  in  transformers  and  iron  cores,  which 
have  the  effect  of  a  small  transformer  interposed  in  the 
circuit.  This  component  of  impedance  is  termed  energy 
resistance,  and  the  other,  reactance  which  has  sometimes 
been  called  the  inductive  resistance. 

Reactance  is  the  effect  of  self-induction  expressed  in 
ohms.  It  becomes  prominent  in  lines  of  large  cross- 
section.  The  relative  value  of  reactance  to  resistance  can 
be  reduced  by  selecting  a  number  of  conductors  of  small 
areas  having  a  combined  equal  resistance.  For  instance, 

when  for  one  No.  ooo 
wire  two  No.  I  wires  are 
substituted,  the  resistance 
will  remain  the  same,  but 
the  reactance  will  be  al- 
most halved. 

When  capacity  is  intro- 
duced into  the  circuit,  the 
current  may  lead  in  phase.  Fig.  6  illustrates  the  effect  of 
capacity  on  the  circuit.  The  reactance  due  to  capacity,  or 
condensance  as  it  is  designated,  acts  in  the  opposite  direc- 
tion to  the  reactance  of  inductance.  The  impedance  in 
Fig.  6  is  the  resultant  of  the  resistance  and  the  capacity  re- 
actance. When  capacity  and  inductance  are  both  present, 
the  impedance  is  the  resultant  of  the  resistance  component 
and  a  component  equal  to  the  difference  between  the  numer- 
ical values  of  the  condensance  and  reactance.  In  Fig.  7 
the  magnetic  reactance  is  laid  off  above  the  line  of  resis- 
tance and  in  quadrature  with  it.  The  capacity  reactance,  or 
condensance,  is  represented  as  having  a  greater  numerical 
value,  and  acting  in  opposing  direction.  The  resultant  im- 
pedance is  readily  found.  When  the  inductance  is  equal 


Fig.   6. 


ALTERNATING-CURRENT   TERMS.  9 

to  the  capacity,  the  current  is  in  phase  with  the  impressed 
volts,  and  follows  Ohm's  law. 

In  aerial  conductors  of  low  resistance,  the  reactance  is 
often  prominent,  and  the  distribution  of  E.M.F.  may  be 
seriously  affected  by  it.  It  becomes  important,  then,  in 
selecting  conductors  for  transmission  lines,  that  those  of 
large  cross-section,  and  correspondingly  low  resistance,  be 
avoided  as  much  as  possible,  except  in  special  cases,  as, 
for  instance,  in  the  employment  of  rotary  converters  sup- 
plied by  its  own  set  of  con- 
ductors, where  some  react- 
ance may  be  desirable. 

Virtual  Resistance.  —  If 
the  cross-section  of  a  con- 
ductor carrying  an  alter- 
nating current  is  resolved 
into  many  elements,  it  will 

be  seen  that  the  internal  portions  are  subject  to  greater 
inductive  effects  than  the  elements  nearer  the  surface. 
The  outer  streams  of  current  suffer  less  opposition,  and 
reach  a  maximum  sooner  than  those  centrally  located. 
In  large  conductors,  carrying  heavy  currents  of  high  fre- 
quency, there  may  not  only  be  no  current  flowing  in  the 
central  portion  of  the  conductor,  but  a  condition  may  exist 
where  a  current  will  flow  in  the  opposite  direction.  The 
central  core  is  then  not  only  valueless  as  a  conductor,  but 
had  better  be  omitted. 

As  a  result  of  the  reduction  of  the  effective  cross-section 
of  a  conductor  carrying  an  alternating  current,  the  resis- 
tance is  increased,  and  slightly  less  current  will  flow  than 
would  if  the  specific  resistance  and  the  inductance  of  the 
wire  are  alone  considered.  This  increment  of  resistance 


10        POLYPHASE  APPARATUS  SYSTEMS. 

of  a  conductor  is  called  its  virtual  resistance.  The  phe- 
nomenon is  also  called  the  skin  effect. 

The  best  shape  for  conductors  of  large  cross-section,  carry- 
ing heavy  alternating  currents,  is  that  of  a  tube  or  flat  strip. 

In  common  practice  the  sizes  of  wire  and  the  rapidity 
of  current  reversals  are  not  such  as  to  appreciably  produce 
this  effect.  The  ratio  of  the  resistance  of  a  conductor 
carrying  an  alternating  current,  to  its  resistance  when  a 
direct  current  is  flowing,  can  be  readily  computed  for  dif- 
ferent sizes  of  conductors  and  reversals  of  current. 

In  Fig.  8  the  ordinates  represent  the  product  of  area 
and  cycles  per  second.  Corresponding  factors  for  the 
virtual  or  apparent  resistance  of  cylindrical  copper  con- 
ductors are  read  off  the  horizontal  scale.  The  ordinate 
for  the  factor  for  a  conductor  of  any  other  non-magnetic 
metal  is  the  product  of  the  ratio  of  its  conductivity  to  that 
of  copper,  and  the  sectional  area  times  the  frequency.  In 
the  case  of  magnetic  materials,  especially  iron,  the  factor 
for  the  virtual  resistance  is  greater  than  that  in  the  curve. 

Energy  in  a  Circuit.  —  The  work  done  in  a  circuit  will 
always  be  some  product  of  the  current  and  the  quantities 
in  phase  with  it.  In  a  direct-current  system  the  product 
of  measured  volts  and  amperes  will  give  the  energy  of  the 
circuit.  In  an  alternating-current  system  the  product  of 
the  measured  amperes,  and  the  component  of  impressed 
E.M.F.  in  phase  with  the  current,  —  i.e.,  the  energy 
E.M.F.,  —  will  give  the  energy.  The  component  of  E.M.F. 
in  quadrature  with  the  current  —  i.e.,  the  induction  com- 
ponent—  drives  a  wattless  current,  and  consequently  adds 
nothing  to  the  energy  of  the  circuit.  The  product  of  the 
impressed  E.M.F.  and  the  current  will  be  easily  under- 
stood to  give  too  large  results.  This  product  is  the 


ALTERNATING-CURRENT   TERMS. 


I  i 


apparent  watts  of  the  circuit.  The  error  in  calculating 
the  power  by  the  measured  amperes  and  volts  will  depend 
upon  the  extent  of  the  displacement  in  phase  of  the  im- 
pressed E.M.F.  and  the  current,  or  the  angle  of  the  lag, 
usually  denoted  as  3>.  The  energy  in  the  circuit  can  be 


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Factor  for  Virtual  Resistance. 
Fig.   8. 

found  by  multiplying  the  product  of  volts  and  amperes  by 
the  cosine  of  this  angle  of  lag. 

Power  Factor  —  Induction  Factor. — The  ratio  of  the 
true  watts  in  the  circuit,  as  measured  by  an  indicating 
wattmeter,  to  the  apparent  watts,  — the  volt-amperes,  —  is 
called  the  power  factor.  The  power  factor  is  useful  in 
determining  the  true  energy  in  a  circuit  when  the  apparent 


12        POLYPHASE  APPARATUS  SYSTEMS 

energy  is  known,  the  resistance  when  the  impedance  is 
known,  the  energy  volts  when  the  total  impressed  volts 
are  given,  and  the  energy  current  when  the  total  current 
is  known. 

The  quantities  in  quadrature  with  the  energy  values  of 
current  and  E.M.F.,  and  with  the  resistance,  may  be  deter- 
mined in  the  same  way,  from  the  resultants  by  a  multiplier 
or  factor,  called  the  induction  factor.  As  the  power  factor 
is  proportional  to  the  energy  quantities,  and  the  induction 
factor  to  the  components  in  quadrature  with  them,  it  fol- 
lows that  the  former  must  be  numerically  equal  to  the 
cosine,  and  the  latter  to  the  sine  of  the  lag  angle.  Ac- 
cordingly, a  table  of  cosines  and  sines  for  all  angles  will 
give  the  corresponding  power  and  induction  factors. 

Wattless  Current.  —  The  component  of  the  total  current 
in  quadrature  with  the  energy  current  is  called  the  watt- 
less current.  It  should  be  understood  that  the  current 
and  other  quantities  of  a  circuit  are  resolved  into  compo- 
nents for  the  sake  of  a  better  understanding  of  the  phe- 
nomena taking  place  in  the  circuit.  There  is  actually  but 
one  current  flowing,  as  there  is  but  one  E.HI.F.,  in  any  one 
part  of  a  circuit.  The  presence  of  reactance,  either  in  the 
transmission  circuit  or  in  the  apparatus  connected  to  it, 
increases  the  lag-angle,  and  consequently  the  wattless 
current.  This  component  does  no  work  in  a  circuit,  but 
increases  the  total  current,  and  thereby  the  heating  of 
conductors.  The  wattless  current  required  to  balance  the 
reactance  may  become  sufficiently  great  to  practically  tax 
the  full  capacity  of  generators  and  of  conductors,  although 
very  little  energy  is  being  generated  or  transmitted.  If  it 
were  possible  to  have  conductors  without  resistance,  a 
true  wattless  current  could  then,  in  fact,  actually  exist  in 


ALTERNATING-CURRENT   TERMS.  13 

an  alternating-current  circuit.  In  such  a  case  the  total 
current  would  be  in  quadrature  with  the  impressed  E.M.F '., 
and  the  circuit  would  give  back  as  much  energy  as  it 
received,  the  sum  being  zero. 

Relative  Values.  —  Designating  the  current  as  7,  resis- 
tance as  R,  reactance  as  S,  and  impedance  as  U,  from 
what  has  preceded,  the  following  relations  will  be  under- 
stood : 

1.  The  reactance  ")         Induction  E.M.F.  consumed  in  line 

of  a  line,  S,    j  7 

r^,      •  Impressed  E.M.F,  consumed  in  line 

2.  I  he  impedance,  t/,  =  —  —  • 

3.  The  energy  component  of  E.M.F.  consumed  by  the  resis- 

tance, R,  of  a  conductor  is  7/v*,  and  is  in  phase  with  the 
current. 

4.  The    inductive    component    of    E.M.F.    consumed    by   the 

reactance,  S,  of  a  conductor  is  IS,  and  is  in  quadrature 
with  the  current. 

5.  The  impressed  E.M.F.  consumed  by  the  impedance,  U,  of  a 

conductor,  is  IU. 

5.    The  energy  loss  in  a  conductor  is  727?,  and  depends  on  the 
current  and  resistance  only. 

Voltage  Drop  Dependent  on  Load  Characteristic.  —  The 
E.M.F.  consumed  by  the  impedance,  IU,  does  not  represent 
the  voltage  drop  in  a  conductor,  as  it  is  usually  out  of  phase 
with  the  impressed  E.M.F.  as  well  as  with  the  current. 
This  voltage  drop,  as  will  be  shown,  can  be  anything  be- 
tween IR  and  IU.  It  will  depend  upon  the  difference  in 
phase  between  the  current  and  the  impressed  •  E.M.F.,  or 
the  lag  angle,  and  can  be  determined  when  the  power 
factor  is  known.  In  Figs.  9  to  14,  let  OE'  be  the  E.M.F. 
a.t  the  receiving  end  of  a  transmission  line.  For  various 


POLYPHASE  APPARATUS  SYSTEMS. 


power  factors  at  the  receiving  end  of  the  line,  there  will  be 
corresponding  phase  differences,  <£.      Let  OI  be  the  cur- 
rent out  of  phase  with  the 
E.M.F.    by   $.     IU,   IR, 
and  75  have  the  relations 
heretofore   assigned  to 
them,  IR  being  in  phase 
with  (97,  and  IS  in  quad- 
S 


(>=90 


Fig.  9. 


rature  with  OI.  Where 
these  quantities  are  small 
relatively  to  the  impressed 
E.M.F.,  as  they  usually 
are  in  practice,  the  drop 

of  voltage  is  IA,  equal  to  OE  —  OE1,  OE  being  equal  to 
the  generator  voltage,  A  the  apparent  resistance  of  the  line. 
Assume  a  given  E.M.F.  at  the  end  of  the  line,  and  a 
constant  resistance  and  reac- 
tance. If  the  phase  displace- 
ment <£,  or  what  is  the  same, 
if  the  power  factor  of  the 
receiving  system,  is  varied, 
the  triangle  of  electromotive 
forces  will  revolve  around  E1 
as  a  center.  The  pro]  action  of 
'IU,  or  its  components,  upon 
the  E.M.F.  will  give  the  vol- 
tage drop.  With  a  lag  angle 
of  90°  (Fig.  9),  the  drop  of  voltage  is  due  to  the  reac- 
tance alone.  As  the  lag  angle  decreases,  the  drop  IA  be- 
comes less  than  the  impressed  E.M.F.  consumed  in  the  line 
IU  until  it  reaches  60°  (Fig.  10),  when  with  the  given 
values  of  IU  and  75  the  drop  is  seen  to  be  equal  to  the 


=60° 


E'     U 


Fig.  10. 


ALTERNATING-CURRENT    TERMS 


Fig.  11. 


impedance  1U,  and  has  the  greatest  value  it  can  have. 
As  the  phase  displacement  grows  less,  the  effect  of  the 
reactance  decreases  until  <£  =  o  (Fig.  11),  when  the  drop 
is  due  to  resistance  alone,  a  case  of  a  non-inductive  load. 

If  capacity  effect  is 
now  introduced  into  the 
line  by  the  use  of  long 
cables,  over  excited  syn- 
chronous motors,  and  ro- 
tary converters,  or  of 

condensers,  the  phase  displacement  $  becomes  negative. 
Up  to  30°  the  projection  of  the  reactance  is  in  opposition 
to  the  projection  of  the  impedance,  i.e.,  negative  (Fig.  12), 

and  as  a  result  the 
E  A  drop  I  A  is  less  than 

o  ^^^  10=20°  r>-^ 

the  resistance  drop. 

Finally,  at  30°  (Fig. 

1 3)  there  is  no  drop 

of  voltage  in  the 
line  ;  for  the  reactance  raises  the  voltage  as  much  as  the 
resistance  lowers  it,  and  the  line  apparently  has  no  re- 
sistance. As  the  phase  displacement  increases,  the  vol- 
tage at  the  receiving 
end  becomes  higher 
than  the  generator 
E.M.F.,  due  to  the 
predominating  effect 
of  the  capacity  reac- 
tance over  the  resis- 
tance. This  is  the  greatest  at  90°  (Fig.  14).  For  the 
sake  of  simplicity  we  have  assumed  in  the  foregoing  that 
the  projection  of  E  determines  the  apparent  resistance. 


Fig.  12. 


Fig.  13. 


i6 


POLYPHASE   APPARATUS    AND    SYSTEMS. 


0=90 


This   is   not   strictly  accurate,   but   in   practice   the   error 
involved  will  be  found  to  be  insignificant. 

Frequency. — The  number  of  complete  reversals  of  alter- 
nating quantities  in  any  given  time  is  called  their  fre- 
quency. Each  complete  reversal  is  a  period  or  cycle,  and 
is  measured  in  degrees.  An  alternation  is  a  half  period 
or  cycle,  and  in  the  curve  of  impressed  E.M.F.  (Fig.  i)  is 

measured  by  the  value 
A      I  of  the  E.M.F.  from  o° 

to  1 80°,  and  from  180° 
to  360°.  In  a  bipolar 
generator  every  revo- 
lution of  the  armature 
corresponds  to  one 
cycle.  In  multipolar 
generators  there  will  be  as  many  cycles  for  every  revo- 
lution as  there  are  pairs  of  poles.  Frequency  is  usually 
denoted  in  cycles  per  second.  In  a  twenty-four  polar 
generator  of  300  R.P.M.,  the  number  of  alternations  per 
minute  is  7,200.  The  number  of  cycles  per  minute  is 
one-half  of  this,  or  3,600,  and  in  one  second  is  60.  Ap- 
plying this,  — 

Frequency,  or    )        Poles  X  R.P.M. 
Cycles  per  sec.  )  60  X  2 


Fig.  14. 


GENERATORS.  1 7 


CHAPTER  II. 
GENERATORS. 

Elementary  Forms.  —  The  simplest  form  of  polyphase 
generator  consists  of  two  single-phase  alternators  coupled 
together  on  one  shaft  in  such  a  manner  that  the  electro- 
motive forces  at  the  terminals  of  the  armature  conductors 
arrive  at  a  maximum  90°,  or  one-fourth  of  a  period,  apart. 
The  currents  from  this  machine  are  said  to  have  a  two- 
phase  relationship.  An  arrangement  of  three  such  arma- 
tures, with  similar  coils  one-third  of  a  pole  arc,  or  60 


Fig.    15. 

electrical  degrees,  apart,  will  generate  three-phase  currents. 
Fig.  1 5  illustrates  the  armature  connections  of  an  ideal 
three-phase  unit,  made  up  of  three  single-phase  alternators 
arranged  in  this  manner. 

This  combination  of  two  or  more  independent  alterna- 
tors, forming  one  polyphase  unit,  facilitates  the  regulation 
of  the  circuits  in  case  of  unbalancing,  as  the  fields  ( not 
shown  in  the  diagram )  are  separate.  This  form  of  gen- 
erator is  not  commercially  manufactured,  as  it  is  naturally 
expensive.  Being  made  up  of  smaller  machines,  the  cost 


1 8     POLYPHASE  APPARATUS  AND  SYSTEMS. 

would  be  greater  than  that  of  a  single  unit  of  the  same 
output.  Polyphase  generators  are  smaller,  and  conse- 
quently cheaper  to  build,  than  single-phase  alternators  of 
the  same  capacity.  Most  types  of  polyphase  generators 
have  one  field  and  one  armature,  with  as  many  sets  of  wind- 
ings as  there  are  phases.  Irregularities  in  the  voltage  of 
the  different  phases  —  if  any  exist  —  must  be  overcome 
in  some  other  manner  than  by  a  variation  of  the  field 
strength.  In  some  inductor  types  of  generators,  this  reg- 
ulation is  obtained  by  varying  the  number  of  armature 
turns  in  the  unbalanced  phase. 

The  principles  of  construction  and  operation  of  single- 
phase  generators  apply  equally  well  to  polyphase  machines. 
The  requirements  of  recent  alternating-current  practice, 
involving  the  transmission  and  utilization  of  power  to  an 
extent  that  has  completely  overshadowed  the  purely  light- 
ing branch  of  the  art,  have  necessitated  vast  improvements 
in  machinery  and  methods.  Not  the  least  improvement  has 
been  in  polyphase  generators. 

Revolving  Armature  Type.  —  A  type  of  alternating- 
current  generator  at  one  time  commonly  employed  in  the 
United  States  is  that  in  which  the  armature  is  the  moving 
member.  For  some  time  past  in  Europe,  and  more  re- 
cently in  this  country,  another  type  has  come  into  almost 
universal  use,  a  type  in  which  the  armature  is  stationary, 
and  the  field  structure  is  the  revolving  part.  All  modern 
generators,  including  what  is  known  as  the  inductor  gen- 
erator, are  modifications  of  either  the  revolving  armature 
or  the  stationary  armature  type. 

Fig.  1 6  illustrates  a  standard  form  of  belt-driven  gen- 
erator of  the  revolving  armature  construction.  The 
frame  has  graceful,  dignified  lines,  the  bearings  form- 


GENERATORS.  19 

ing  one  casting  with  the  lower  yoke  and  base.     The  pole- 
pieces  project  inwardly  from  the  frame,  and  are  made  up 


Fig.    16. 


of  steel  laminations  cast  into  the  yoke.     The  field  coils  are 
wound  upon  insulated  spools,  and  are  removable.     When 


20 


POLYPHASE    APPARATUS   AND   SYSTEMS. 


these  generators  are  built  for  automatic  compounding,  two 
field  windings,  one  for  the  separate  and  one  for  the  self- 
exciting  current,  are  required.  The  armature  is  of  the  iron- 
clad type,  and  is  built  up  of  laminations,  slotted  to  admit  the 
coils.  These  are  usually  machine-wound,  and  held  firmly  in 
place  by  seasoned  wedges  of  wood.  This  armature  winding 
construction,  and  the  finished  core  and  shields  of  an  arma- 


Fig.    17. 

tare,  are  shown  in  Fig.  17.  An  injury  to  the  insula- 
tion, from  lightning  or  other  causes,  is  usmlly  limited 
to  one  or  a  few  adjacent  coils,  which  can  be  easily 
replaced  without  disturbing  the  rest  of  the  winding. 
All  the  standard  belted  polyphase  generators  of  the  revolv- 
ing armature  type  conform  to  the  general  lines  of  the 
generator  shown  in  Fig.  16.  Generators  of  an  output 


GENERATORS.  21 

greater  than  200  K.W.  are  usually  provided  with  a  third, 
or  outboard,  bearing  to  sustain  the  weight  of  the  pulley 
and  strain  of  the  belt.  Generators  of  500  K.W.,  and  over, 
are  almost  invariably  built  for  direct  connection  to  either 
engine  or  water-wheel.  If  built-  for  connection  to  the 
former,  the  base  is  ordinarily. omitted,  while  generators  for 
coupling  to  water-wheels  are  provided  with  base  and  two 
bearings,  and  in  small  sizes  are  self-contained  as  a  rule, 
the  base  and  two  bearings  comprising  one  casting.  When- 


Fig.    18. 

ever  possible,  a  generator,  irrespective  of  its  size,  should 
be  direct-connected,  on  account  of  saving  of  space  and 
of  belt  losses. 

Revolving  Field  Type. —  The  revolving  field  type  of 
generator  is  one  of  a  number  of  forms  of  the  stationary 
armature  machine.  The  rotating  member,  or  field,  con- 
sists of  a  heavy  cast-steel  wheel,  into  which  are  bolted  or 
keyed  pole-pieces,  projecting  radially  outward.  These  are 
usually  built  up  of  laminated  sheet-iron.  Fig.  18  illus- 
trates a  rotating  field  magnet  of  a  200  K.  W.  generator. 


22 


POLYPHASE   APPARATUS   AxND    SYSTEMS. 


The  laminated  construction  prevents  formation  of  eddy 
currents,  which  would  occur  if  the  pole-pieces  were  solid 
castings.  The  coils  are  wound  on  spools,  placed  on  the 
poles,  and  held  in  place  by  the  pole-tips.  The  field 
coils  on  large  machines  are  made  of  a  single  spiral  of 
strip-copper,  wound  on  edge.  Fig.  19  shows  the  con- 
struction of  the  field-spools  of  a  750  K.W.  generator. 
On  small  machines  wire  is  used.  The  revolving  field 
acts  like  a  fan,  forcing  the  air  outwardly  through  the 


Fig-.  19. 

openings  between  the  armature  laminations.  The 
shields  of  the  circular  armature  structure  prevent  un- 
due loss  through  the  windage  of  the  revolving  field. 
Direct  current  for  excitation  is  carried  to  the  field  by 
means  of  a  small  two-ring  cast-iron  or  copper  collector, 
equipped  with  carbon  brushes,  requiring  practically  no 
attention  in  operation. 

The  stationary  armature  consists  of  a  circular  cast-iron 
frame  or  spider,  inside  of  which  are  dove-tailed  sheet- 
iron  disks,  with  slots  to  receive  the  coils.  Ventilating 


GENERATORS.  23 

spaces  are  left  between  laminations,  through  which  the 
air  flows  rapidly  when  the  generator  is  running.  Fig.  20 
shows  the  construction  of  a  stationary  wire  wound  three- 
phase  armature  of  750  K.W.  capacity. 


Fig.  20. 

Fig.  21  is  a  sectional  view  of  the  field  and  armature 
of  a  typical  revolving  field  three-phase  generator.  The 
relation  of  the  magnetic  circuit  to  the  armature  coils  is 
clearly  shown. 


24 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


The  generator  shown  in  Fig.  22  is  one  of  a  number 
installed  near  Redlands,  Cal.  It  has  20  poles,  and  runs 
at  300  R.P.3L,  giving  a  current  of  50  cycles.  The  driv- 
ing-power of  each  generator  is  supplied  by  Pelton  water- 
wheels,  keyed  to  the  shaft,  and  mounted  and  housed  on 
the  generator  base,  as  indicated.  The  machine  is  wound 
for  750  volts,  no  load,  and  generates  in  each  branch 
525  amperes.  The  commercial  efficiency  at  full  load  is 


•Fig.   21. 

95.6  per  cent.  The  regulation  on  non-inductive  load  is 
7.  i  per  cent.  The  cut  shows  the  armature,  slid  along 
on  its  base  to  permit  ready  inspection  of  the  field  and 
other  parts. 

Fig.  23  shows  a  Ganz  &  Co.  80  K.W.  revolving  field 
generator.  The  armature  winding  used  in  this  machine 
is  in  the  form  of  spools  bolted  to  the  outer  ring.  This 
arrangement  has  the  advantage  of  accessibility  for  inspec- 
tion and  repair.  The  field  construction  is  practically  the 


GENERATORS. 


26 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


same  as  that  of  the  generator  described  above.  It  is 
claimed  that  these  machines  have  a  moderate  armature 
reaction,  and  at  the  same  time,  when  short-circuited,  will 


Fig.  23. 

not  deliver  more  than  two  and  one-half  times  the  normal 
full-load  current. 

Another  form  of  the  stationary  armature  type  of  gen- 
erator is  one  in  which  the  field  winding  is  a  single  coil. 
The  exciting  coil  is  wound  on  a  bobbin,  occupying  a 


GENERATORS.  2^ 

channel  on  the  periphery  of  a  cast-iron  wheel.  Two  steel 
rims  are  bolted  to  this,  the  laminations  being  formed 
into  poles.  This  is  one  of  the  original  forms  of"  polyphase 
generators  ;  and  this  construction,  which  has  considerable 
merit,  was  adopted,  in  the  early  days  of  power  transmission 
apparatus,  by  European  manufacturers. 

Inductor  Type.  —  Another  modification  of  the  stationary 
armature  type  is  the  inductor  machine,  manufactured  to 
some  extent  abroad  by  Mordey,  Thury,  and  the  Allgemeine 
Elektricitats  Gesellschaft,  and  in  this  country  chiefly  by 
the  Stanley  Electric  Company.  The  distinguishing  char- 
acteristic of  this  type  is,  that  any  one  set  of  armature  coils, 
or  portion  of  the  armature  conductors,  is  subjected  to  a 
magnetic  flux  of  one  polarity  only.  The  magnetism  fluc- 
tuates from  zero  to  maximum,  and  back  again,  and  does 
not  reverse  its  sign.  Most  generators  of  this  type  have 
both  fixed  armature  and  fixed  field-windings,  the  only 
moving  part  being  the  inductor,  —  a  laminated  iron  core, 
with  polar  projections.  The  exciting  winding,  wound  into 
an  annular  coil,  is  sometimes  placed  centrally  on  the 
internal  surface  of  the  armature  spider,  embracing  the 
revolving  element,  as  in  the  Stanley  machine.  This  is 
usually  a  ring  of  iron,  with  a  double  row  of  laminated 
polar  projections.  In  some  machines,  notably  those  made 
by  Thury  abroad  and  by  the  Warren  Company  of  San- 
dusky,  Ohio,  and  the  Westinghouse  Company,  the  arma- 
ture has  a  single  set  of  coils,  and  the  inductor  is  provided 
with  a  single  row  of  laminations.  The  annular  exciting 
coil  may  be  part  of  the  revolving  element,  and  revolve 
with  it. 

Reference  to  Fig.  24  will  show  the  general  arrangement 
of  the  magnetic  circuit  of  the  Stanley  inductor  generator. 


28  POLYPHASE    APPARATUS    AND    SYSTEMS. 


GENERATORS.  29 

The  annular  field  coil,  Fy  is  surrounded  by  the  magnetic  cir- 
cuit, made  up  of  the  laminated  cores  AA,  the  armature 
yoke  Yv  and  the  laminated  poles  JV  and  S,  and  the  field 
yoke  F2.  The  armature  windings,  consisting  of  two  com- 
plete sets,  are  laid  in  grooves  in  the  armature  cores  in  a 
manner  similar  to  the  revolving  field-machine.  It  will  be 
seen  that  the  North  and  South  poles  do  not  alternate,  but 
the  magnetic  flux  simply  pulsates  in  one  direction.  Only 
one-half  of  each  turn  of  the  armature  winding  is  in  an 
active  field  at  one  time,  the  other  half  of  the  coil  being 
between  the  poles  in  an  inactive  field.  The  E.M.F.  gene- 
rated is  one-half  as  great  as  it  would  be  if  the  polarity 
of  the  flux  were  reversed.  In  order  to  obtain  a  given 
E.M.F.  with  the  inductor  type  of  machine,  either  the  arma- 
ture windings  or  the  total  magnetic  flux  must  be  doubled. 
The  essential  characteristics,  therefore,  of  an  inductor 
generator  are  a  rather  high  density  of  the  magnetic 
circuit,  and  a  short  air  gap,  the  latter  in  order  to  reduce 
the  magnetic  leakage  to  a  minimum.  The  stationary  ele- 
ment of  the  Stanley  inductor  machine  consists  of  two 
series  windings,  forming  two  separate  armatures.  The 
currents  in  the  coils  are  usually  in  quadrature  with  each 
other,  thus  giving  a  two-phase  current.  A  three-phase 
relationship  can  be  established  by  means  of  a  symmetrical 
three-phase  winding,  or  by  making  one  set  of  coils  with 
.86  the  number  of  turns  of  the  other,  and  connecting  the 
end  to  the  middle  of  the  larger  coil.  By  the  theory  of 
the  resultant  of  electromotive  forces,  the  currents  in  the 
three  circuits  will  be  equal,  and  the  impulses  will  follow 
one  another  at  intervals  of  60°.  Fig.  25  shows  a  600 
K.W.  Stanley  inductor  generator. 

Fig.  26  shows  a  sectional  view  along  the  shaft  of  an 


30  POLYPHASE    APPARATUS    AND    SYSTEMS. 

inductor  generator  manufactured  by  the  Warren   Electric 
Manufacturing  Company.      GG  is  the  frame,  or  spider,  of 


*  •••* 


the    stationary   armature,   into   which    are   dovetailed    the 
laminated    polar    projections  AA.      CC  are    the  armature 


GENERATORS.  31 

coils  surrounding  the  poles.  The  revolving  element  is 
made  up  of  the  spider  H  carrying  the  laminated  polar  pro- 
jections DD.  F  is  a  single  magnetizing  coil.  The  mag- 
netic circuit  is  from  G,  through  A,  to  D,  and  thence  from 
H  to  G.  It  will  be  seen  that  there  are  two  air  gaps,  one 
between  A  and  D,  and  the  other  between  G  and  H.  As 
in  all  inductor  generators,  the  magnetism  pulsates  only, 
and  the  revolving  polar  projections  have  one  polarity. 


Fig-.  26. 


&    armature,    is 
distribution  where 


The  type  of  generator,  with  revolving: 
confined  to  a  general  power  and  lightin 
only  a  moderate  voltage  is  required.  Machines  of  this 
type  are  cheap  to  build.  They  can  be  automatically  com- 
pounded, without  any  complication  of  parts,  which  is  not 
the  case  in  the  revolving  field  or  inductor  generator. 

This   construction   is  not  suitable  for  high   or   for   low 
voltages,  on  account  of   the  difficulties  of  insulating  the 


POLYPHASE   APPARATUS    AND    SYSTEMS. 


collector  rings  in   the  first   case,  and  of  collecting  a 
current  in  the  second  case. 

The  stationary  armature  can  be  easily  insulated  to  with- 
stand a  testing  pressure  of  over  30,000  volts;  and,  as  no 
collecting  device  is  required,  currents  of  any  volume  can 
he  cared  for. 

(Generators  with  stationary  armatures  are  now  wound 
for  pressures  up  to  20,000  volts,  hut  it  is  doubtful  whether 
it  is  economical  or  advisable  to  wind  them  for  more  than 
15,000  volts. 

Armature  Windings.  —  For  details  of  windings  of  gen- 
erator armatures,  the  reader  is  referred  to  more  compre- 
hensive works  on  the  subject.  The  armature  windings  of 
polvphase  generators  are  composed  of  two  or  more  groups 
of  single-phase  windings  suitably  connected  to  give  the 
desired  phase  relations.  The  armature  windings  of  mod- 
ern alternators  are  laid  in  slots  or  grooves,  below  the  sur- 
face of  the  armature  punchings.  The  shape  and  number 
of  the  slots  have  a  material  effect  upon  the  performance 

of  a  generator,  as  we 
will  proceed  to  show. 
The  old-fashioned  iron- 
clad armature  had  one 
coil  for  each  pole,  or 
pair  of  poles,  laid  in 
deep  slots.  On  account 
of  this  grouping  of  the 
conductors  into  a  coil 
of  many  turns,  this  gen- 
erator possessed  high  armature  reaction,  and  could  be 
short-circuited  with  no  bad  effects.  This  construction  is 
sometimes  carried  out  in  those  modern  polyphase  genera- 


27. 


GENERATORS.  33 

tors  whose  armatures  have  one  slot  for  each  phase  and 
each  pole,  and  are  called  unilooth  machines.  Thus  the 
armature  of  an  eight-pole  two-phase  generator  has  8  coils  ; 
a  three-phase  generator  of  the  same  number  of  poles  has 
12  groups  of  conductors.  The  shape  of  the  armature 
punchings  of  a  12-pole  unitooth  three-phase  generator  is 
shown  in  Fig.  27.  Sometimes  the  laminations  have  cir- 
cular holes  instead  of  slots.  The  advantage  of  safety,  in 
case  of  short  circuits,  is  a  doubtful  one,  as  most  plants  are 
provided  with  protective  devices  which  render  a  short  cir- 
cuit more  inconvenient  than  dangerous.  Armature  reac- 
tion deforming  the  wave- 
shape of  the  /:Jf.F., 
and  high  inductance,  re- 
quiring large  exciting 
currents  at  full  load, 
are  often  characteristic 
of  the  unitooth  winding. 
As  will  be  shown,  these 

Fig.  28. 

generators    can    be    de- 
signed  so   as   in   a  great   measure  to  overcome  these  ob- 
jections. 

Many  modern  polyphase  armatures  have  two  or  three 
slots  per  pole  per  phase.  The  slots  are  open,  which,  with 
the  distributed  form  of  winding,  gives  a  very  low  induc- 
tance (Fig.  28).  This  necessitates  only  a  slight  increase 
of  exciting  current  at  full  load.  Generators  with  multi- 
tooth  armatures  are  most  suitable  for  long-distance  trans- 
mission, where  step-up  transformers  are  employed.  The 
regulation  is  good,  and  the  wave-shape  approaches  a  sine- 
curve, —  the  best  shape  for  this  work,  as  it  reduces  the 
possibility  of  resonance,  or  rise  of  voltage,  at  a  distant 


34    POLYPHASE  APPARATUS  AND  SYSTEMS. 

point  in  the  transmission  circuit,  above  that  at  the  gen- 
erating end. 

The  various  connections  of  generator  armature  windings 
will  be  found  explained  in  chapters  on  polyphase  systems. 

Electromotive  Force The  drop  at  the  terminals  of  a 

direct-current  generator,  as  the  output  is  increased,  is 
principally  due  to  the  armature  resistance  and  reaction. 
In  alternators  the  IR  drop  is  generally  not  so  prominent 
as  that  due  to  inductance  and  to  armature  reaction.  The 
counter  E.M.F.  of  self-induction  lowers  the  terminal 
pressure,  and  armature  reaction  by  opposing  its  flux  to 
the  field  magnetism  reduces  the  effective  number  of  lines 
of  force,  passing  through  the  armature  conductors,  with 
the  like  result. 

The  inductance  of  unitooth  armatures  can  be  lessened 
by  widening  the  opening  of  the  slots,  which,  at  the  same 
time,  increases  the  resistance  to  the  magnetic  flux,  —  i.e., 
the  reluctance  of  the  air  gap.  As  inductance  varies, 
directly,  with  the  square  of  the  number  of  turns,  by  using 
fewer  turns  per  slot  and  more  slots,  —  in  other  words,  the 

distributed  form  of 
B  winding,  —  this 

property  can  be 
much  reduce  cl, 
without  sacrificing 
efficiency,  or  in- 
creasing the  cost 
Fig.  29.  of  the  generator. 

Armature  reac- 
tion is  greatest  when  the  load  is  inductive,  as  then  the 
current  lags  behind  the  E.M.F.,  and  brings  the  maximum 
armature  magnetism  in  the  most  favorable  position  for 


GENERATORS 


35 


demagnetizing   the  field.      The  distributed  winding    mini- 
mizes the  evil  effect  of  a  lagging  current.     As  armature 


\ 


\ 


Fig-.  30. 

reaction   produces  a    distortion   of    the    field,    a   curve    of 
E.M.F.,  that  may  be  a  sine-curve  at  no  load,  will  often 


\ 


Fig-.  31. 

depart  widely  from  this  form  when  the  generator  is  loaded. 
The  distortion  of  the  wave-shape  in  unitooth  machines  may 


36         POLYPHASE   APPARATUS    AND    SYSTEMS. 

be  overcome,  in  great  part,  by  careful  shaping  of  the  pole 
pieces. 

While  the  armature  reaction,  due  to  a  lagging  current, 
lowers  the  terminal  E.M.F.  of  a  generator,  a  leading  cur- 
rent may  have  the  opposite  effect,  by  adding  its  flux  to 
that  of  the  field. 

The  relation  between  the  induced  E.M.F.  in  the  arma- 
ture windings  and  the  terminal  E.M.F.  of  a  three-phase 
machine  is  shown  in  Fig.  29.  Curves  A  and  B  represent 
the  voltages  measured  between  the  common  center  and  end 
of  the  armature  coils.  Curve  C,  formed  by  uniting  these 
Y  electromotive  forces,  is  the  A  E.Hf.F.  or  pressure  be- 
tween the  terminals  of  the  armature  coils,  and  therefore 
the  measured  line  voltage.  In  this  way,  if  the  line  voltage 
is  found  to  be  1,732  volts,  the  voltage  of  any  conductor 

I   732 

with  respect  to  the  common  center  is  -       -  =  1,000. 

V3 

The  Y  E.M.F.  of  a  standard  three-phase  unitooth  ma- 
chine under  full  load  is  shown  in  Fig.  30. 

The  "delta  "  E.Jlf.F.,  or  curve  of  pressure  between  any 
two  of  the  line  wires  under  the  above  condition  of  load,  is 
shown  in  Fig.  31.  This  last  curve  can  be  readily  obtained 
by  uniting  the  Y  curves  for  any  particular  condition  of 
load,  displaced  60°. 


GENERATORS. 


CHAPTER    III. 
GENERATORS   (CONCLUDED). 

Field  Excitation  and  Compounding.  —  The  voltage  of  a 
generator  may  be  maintained  uniform,  under  all  normal 
conditions  of  load,  by  varying  the  strength  of  the  field 
excitation.  For  local  lighting  and  power  distribution 
where  the  circuits  have  fairly  equal  loads,  an  automatic 
or  compounding  arrangement,  ;as  it  is  called,  is  generally 
desirable.  The  same  results  are  obtained,  in  a  measure, 
by  using  generators  of  good  regulation  and  proper  fre- 
quency, and  sufficient  line-copper  to  keep  the  loss  down 
to  within  a  very  few  per  cent.  Generators  of  greater 
capacity  than  300  K.VV.  are  not,  as  a  rule,  automatically 
compounded.  Generators  for  long-distance  transmission 
work  are  also  without  this  device ;  for,  besides  being 
usually  of  large  capacity,  and  operating  in  parallel,  they 
are  required  to  take  care  of  heavy  voltage  drops  in  the 
transmission  apparatus,  and  of  pressure  variations  due 
to  gradually  changing  loads.  These  voltage  changes 
can  best  be  overcome  by  hand  regulation  of  the  field 
excitation. 

One  method  for  producing  automatic  compounding 
by  variation  of  the  field  excitation  requires  two  sets  of 
field  windings,  —  a  shunt  winding  for  the  current  from 
an  outside  source,  and  a  series  winding  for  the  current 
obtained  from  the  commutation  of  the  alternating  currents 


POLYPHASE    APPARATUS    AND    SYSTEMS. 


of  the  various  phases.  The  connections  of  a  three-phase 
generator  with  composite  field  windings  is  shown  in  Fig. 
32.  A  three-part  commutator  rectifies  the  currents  from 
each  of  the  three-phase  circuits,  so  that  unbalancing 
in  any  one  line  has  a  minimum  effect  on  the  regu- 
lation. The  rotating  shunt  is  practically  the  common 
center  of  the  coil,  giving  a  current  in  the  series  field,  due 

to  about  i  per  cent 
of  the  terminal  vol- 
tage. The  stationary 
shunt  is  adjustable, 
and  can  be  varied  for 
loads  of  different 
power  factors.  It  also 
serves  to  prevent 
sparking  at  the  com- 
mutator. The  con- 
nections of  the  mono- 
cyclic  generator  are 
similar,  except  that 
the  commutator  recti- 
fies the  current  of  the 
main  circuit  only.  In 
another  polyphase 
generator  (Fig.  33), 
the  low  potential  current  for  the  series  field  is  derived 
from  a  series  transformer  within  the  armature.  All 
phases  are  represented  in  the  primary.  The  compounding 
field  current  depends  upon  the  sum  of  the  currents  flow- 
ing in  the  circuits  supplied  by  the  armature. 

The  demagnetizing  effect,  and  consequent  reduction  of 
voltage,  due  to  a  load  of  poor  power  factor,  has  been  ex- 


Fig.  32. 


GENERATORS. 


39 


plained.  A  generator  so  loaded  requires  a  greater  field 
excitation  than  when  running  on  non-inductive  load.  The 
comparative  voltages  with  loads  of  varying  power-factor, 
and  the  same  excitation  is  shown  in  Fig.  35.  Curve 
a  is  the  compounding  when  lights  are  the  chief  load, 
and  b  the  curve  when  the  load  consists  chiefly  of  motors. 


Auxiliary  Field 


Fig-.  33. 

It  will  be  seen  that  a  generator  properly  over-com- 
pounded for  a  night  load  of  lamps  will  not  give  the 
proper  voltage  for  a  day  load  of  motors.  The  stationary 
shunt  in  Fig.  32  will  then  have  to  be  adjusted  for  the  vary- 
ing character  of  the  load.  The  compensated  and  mono- 
cyclic  generators  are  exceptions,  in  that  this  adjustment 


40          POLYPHASE   APPARATUS    AND    SYSTEMS. 

is  not  necessary,  as  will  be  shown  later.  Automatic  com- 
pounding may  also  be  attained  by  variation  of  the  current 
in  the  shunt  windings  alone.  In  this  method  the  exciter 
E.M.F.  is  varied  by  an  arrangement  of  solenoids  and 
magnetic  plungers,  acting  on  the  exciter  rheostat. 

The  energy  required  to  excite  the  fields  of  good  com- 
mercial generators  on  a  non-inductive  load  varies  from 
about  i  per  cent,  in  the  case  of  generators  of  500  K.W. 
capacity  and  over,  to  2,  and  sometimes  3,  per  cent  in 


Kilowatts  Output 
Fig.  34. 

smaller  machines.  The  dynamo  supplying  the  separate 
exciting  current  must,  of  course,  be  of  greater  capacity 
when  the  alternating  generator  is  non-compounded,  and 
does  not  furnish  a  portion  of  the  exciting  current. 

The  exciting  dynamos  are  often  driven  from  a  pulley  on 
the  shaft  of  the  alternating  generator.  In  large  water- 
power  plants  the  best  practice  is  to  drive  the  exciters  from 
separate  water-wheels,  and  in  steam  plants  from  separate 
engines  when  starting  up  and  then  by  motors.  By  this 
method  any  variation  in  the  generator  speed  is  without 
effect  on  the  exciting  current. 


GENERATORS.  41 

An  alternator  containing  some  novel  features  has  re- 
cently been  brought  out,  to  which  the  name  of  "  Compen- 
sated "  alternator  has  been  given.  This  machine  has  the 
appearance  and  all  the  characteristics  of  the  revolving 
field  type,  and  embodies  a  new  method  of  compounding, 
by  which  the  potential  for  all  variations  and  degrees  of  in- 
ductive and  non-inductive  load  are  automatically  adjusted. 
The  means  by  which  this  result  is  accomplished  are  as  fol- 
lows :  the  exciter  is  on  the  same  shaft  with  the  alternator 
revolving  field,  and  has  the  same  number  of  poles,  or  is 
geared  in  such  relationship  to  the  revolving  field  that  the 


Fig-.  35. 

two  operate  synchronously.  In  addition  to  the  exciter 
commutator  and  the  pair  of  collector  rings  which  deliver 
current  to  the  field  of  the  alternator,  the  shaft  carries  three 
collector  rings  (see  Fig.  35),  which  are  connected  to  taps 
in  the  exciter  winding  in  the  same  manner  as  collector 
rings  are  connected  to  the  winding  of  rotary  converters. 
The  current  passes  to  the  collector  rings  from  one  or  sev- 
eral series  transformers  inserted  in  the  main  alternator 
circuit,  which  circuit,  passing  through  the  exciter  arma- 
ture, reacts  magnetically  upon  the  exciter  field,  in  propor- 
tion to  the  strength  and  to  the  phase  relation  of  the  main 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


.IN-    N>X 


alternating  current.  Consequently  the  magnetic  field,  and 
hence  the  voltage  of  the  exciter,  are  due  to  the  combined 
effect  of  the  exciter  shunt  field  and  the  magnetic  effect 
of  the  alternating  current  ;  and  therefore  the  exciter  vol- 


GENERATORS.  43 

tage  rises  not  only  as  the  non-inductive  load  increases,  but 
also  with  additions  of  inductive  load.  The  connections 
between  the  armatures  of  the  two  machines  are  such  that 
the  polarities  induced  in  the  exciter  armature  are  some- 
what ahead  of  the  exciter-field  magnetism,  but  yet  assist 
the  field  magnets.  The  result  is  that  with  increasing 
main  current,  the  current  passing  through  exciter  arma- 
ture strengthens  the  exciter  fields,  and  thereby  the  fields 
of  the  generator.  If  the  current  lags,  as  under  an  induc- 
tive load,  the  magnetization  produced  by  the  alternating 
current  in  the  armature  comes  more  nearly  in  phase  with 
the  exciter  magnetism,  thus  strengthening  the  field  more 
with  a  given  line  current.  The  connections  of  a  three- 
phase  machine  of  this  design  are  shown  in  Fig.  36. 

Regulation Inherent  regulation  is  defined  in  four  or 

five  different  ways ;  but  the  now  commonly  accepted  defi- 
nition is  the  percentage  rise  of  the  voltage  when  full  non- 
inductive  load  is  thrown  off,  the  generator  speed  and  the 
field  excitation  remaining  constant.  From  what  has  been 
said  in  connection  with  armature  windings,  it  follows  that, 
as  a  rule,  generators  with  unitooth  armatures  will  not  have 
as  good  a  regulation  as  the  multitooth  type.  However, 
good  regulation  in  these  machines  can  be  obtained  at  a 
slight  sacrifice  of  efficiency,  or  by  using  more  copper  in  the 
construction  of  the  generator,  and  thus  increasing  its  cost, 
or  by  the  use  of  a  high  magnetic  saturation  of  the  iron. 
A  certain  three-phase  unitooth  machine  of  large  output 
gave  a  regulation  of  6|  per  cent,  from  full  load  to  10  per 
cent  of  the  load.  The  same  generator,  when  the  load  in 
one  circuit  was  reduced  50  per  cent,  did  not  rise  in  voltage 
more  than  5*  per  cent  ;  and  with  no  load  on  one  of  the 
circuits,  the  others  being  fully  loaded,  the  greatest  varia- 


44    POLYPHASE  APPARATUS  AND  SYSTEMS. 

tion  was  8  per  cent.  The  standard  belt-driven  machines 
of  the  unitooth  construction  regulate  within  8  per  cent, 
which  is  close  enough  for  satisfactory  results  to  be  ob- 
tained, even  without  automatic  compounding.  Generators 
of  the  multitooth  construction  require  less  compounding. 
The  standard  belt-driven  machines  of  this  type  have  a 
regulation  of  6  per  cent  or  thereabouts. 

On  inductive  loads  the  regulation,  of  course,  is  not  so 
good.  The  generator  mentioned  above  as  having  a  non- 
inductive  regulation  of  6{  per  cent,  will  require  nearly 
1,200  more  ampere  turns  in  the  field  to  give  full-load  vol- 
tage when  it  is  supplying  current  to  motors,  the  power 
factor  of  the  circuit  being  80  per  cent.  The  regulation 
under  these  conditions  is  about  16  per  cent.  These 
results  are  immensely  superior  to  those  obtained  with  the 
old  iron-clad  alternators,  which  often  required  30  to  50 
per  cent  increase  in  exciting  current  on  non-inductive  loads 
to  maintain  constant  pressure. 

The  construction,  resulting  in  poor  regulation,  is  some- 
times used  in  generators  designed  for  special  purposes  ; 
for  instance,  in  alternating  arc  lighting  where  a  constant 
current  is  required.  Generators  of  a  high  inherent  regu- 
lation are  sometimes  used  in  certain  kinds  of  electric 
smelting,  where  a  constant  watt  output  is  required.  The 
process  is  started  at  a  certain  voltage  ;  and,  as  the  resist- 
ance decreases,  the  voltage  falls  in  inverse  ratio  to  the 
increase  of  current. 

Efficiency  and  Losses.  —  Fig.  37  gives  the  efficiency 
curves  of  a  750  K.W.  three-phase  generator,  and  shows  the 
individual  losses  in  the  machine.  It  will  be  noted  that 
the  highest  efficiency  is  reached  a  little  above  full  load, 
the  losses  being  only  about  5 1  per  cent  of  the  total  out- 


20     40 


100    120 
Fiff.  37, 


140    160    180    200   220 


46          POLYPHASE   APPARATUS    AND    SYSTEMS. 

put.  The  efficiency  at  half  load,  91  per  cent,  is  most 
excellent.  The  friction  loss  is  mainly  due  to  two  bearings, 
and  is  constant  at  about  I  per  cent  for  all  loads.  The 
/.2A'.  loss  in  the  field  varies  little  from  no  to  full  load, 
showing  that  the  generator  is  easy  to  regulate.  The  core 
loss  varies  from  19  K.W.,  at  no  load,  to  24  K.W.,  at  full 
load.  Generators  for  engine  connection  wall  have  an 
apparently  higher  efficiency,  especially  at  light  leads,  as 
the  friction  losses  are,  as  a  rule,  reduced  by  the  omission 
of  all  bearing  losses,  these  being  considered  as  among  the 
engine  losses. 

The  efficiencies  of  generators,  as  usually  given,  do  not 
include  the  losses  in  the  exciter.  As  the  exciter  efficiency 
is  from  80  to  90  per  cent,  and  the  field  loss  about  i  per 
cent,  the  reduction  of  the  generator  efficiency  due  to  this 
source  will  seldom  be  greater  than  .2  per  cent. 

Power  Factor.  —  Manufacturers'  statements  as  to  regu- 
lation and  temperaure  of  generators  are  usually  given  on 
the  basis  of  100%  power  factor  ;  i.e.,  100%  energy  load. 
Generators  are  constructed  with  reference  to  operation  at 
the  full  rated  current  and  voltage  ;  i.e.,  voltampere  output, 
at  power  factors  as  low  as  80%  (80%  energy  load).  When 
operating  at  lower  than  100%  power  factor,  the  regulation 
is  not  as  good.  The  regulation  of  standard  alternators  for 
rated  voltampere  output,  Sof/c  po\ver  factor,  is  approxi- 
mately :  - 

25%  in  cases  where  it  is  stated  as  10%  at  100%  P.F. 

22%    «         "  "  "  «  "       8%  •«       « 

18%  "      "  "         "         "         "     6%  "     " 

The  commercial  efficiency  of  the  average  generator  is 
about  i%  less  for  the  80%  power-factor  load  than  for  the 
1 00%  power-factor  load,  both  rated  in  voltamperes. 


GENERATORS.  47 

The  result  of  operating  at  lower-power  factor  is  to 
increase  the  heating  of  the  field  coils  due  to  the  greater 
excitation  required.  This  increase  at  80%  power  factor 
is  about  5  degrees  C.  The  temperatures  as  given  for 
1 00%  power  factor  are  applicable  to  all  parts  of  the  sev- 
eral machines  except  to  the  field  coils  in  the  case  of 
revolving  field  machines. 

Speed.  — 'As  the  frequency  of  an  alternating-current 
generator,  with  a  given  number  of  revolutions  per  minute, 
determines  its  number  of  poles,  it  follows  that  a  high-fre- 
quency generator,  operating  at  a  normal  speed  of  from,  say, 
300  to  600  R.P.M.,  requires  numerous  poles.  The  old- 
style  alternators  of  moderate  output,  and  of  frequencies  of 
125  to  133  cycles,  ran  at  from  1,500  to  2,000  revolutions, 
and  had  10  to  8  poles.  To  maintain  this  high  frequency,  and 
reduce  the  speed,  thereby  increasing  the  number  of  poles, 
results  ill  an  expensive  and  inefficient  machine.  A  much 
better  machine,  having  a  speed  of  300  or  600  revolutions, 
is  obtained  by  reducing  the  poles  to  24,  or  1 2,  giving  a  fre- 
quency of  60  cycles.  The  majority  of  polyphase  belt- 
driven  generators  in  actual  operation  are  wound  for  60 
cycles.  Standard  belt-driven  generators  of  this  frequency 
have  the  following  number  of  poles  and  speeds  for  the  re- 
spective outputs  : 

K.W.  POLES.  R.P.M. 

5°  9°° 

75  8                           9°° 

100  900 

100  10                              720 

150  12  600 

250  16  450 

500  24  300 


.}8  POLYPI  I  ASK    APPAKATl'S    AM)    SYSTK.MS. 

The  alternating-current  generator  is  far  from  being  like 
a  direct-current  machine,  a  flexible  piece  of  apparatus 
in  respect  to  speed.  The  speed  cannot  be  altered  more 
than  10  per  cent  either  way  from  that  for  which  it  was 
designed,  without  appreciably  affecting  the  constants  of 
the  generator  and  the  apparatus  to  which  the  generator  is 
supplying  current. 

Parallel  Running.  -  In  modern  alternating-current 
plants  of  large  capacity,  especially  long-distance  powei 
transmission  plants,  parallel  operation  is  necessary  in 
order  to  effect  a  reduction  in  the  number  of  circuits  and 
transmission  lines.  Other  advantages  arc1  economy,  sim- 
plicity, and  reliability  of  operation.  Polyphase  generators, 
as  now  designed,  can  be  operated  in  multiple  without  any 
difficulty. 

The  principal  requirement  in  the  generators  is  that  they 
shall  have  a  moderate  armature  impedance  and  a  uniform  ail 
gap.  Too  small  an  impedance  permits  an  excessive  exchange 
of  current  with  slight  inequality  of  the  field  excitation  of  the 
machine,  and  a  dangerous  flow  if  the  generators  are  con- 
nected up  when  they  are  not  quite  in  step.  Generators  hav- 
ing a  large  armature  impedance  will  operate  in  parallel;  but, 
owing  to  the  small  synchronising  current  that  can  be  ex- 
changed, the  condition  is  not  stable,  and  the  generators  are 
liable  to  alternately  lead  in  speed  or  "hunt."  When  a  num- 
ber of  generators  are  to  be  run  in  parallel  the  excitation  of 
each  one  should  be  separately  adjusted  to  give  the  same 
current,  otherwise  there  will  be  an  exchange  of  current. 

The  requirements  of  the  prime  mover  are  uniform  speed 
and  uniform  angular  rotation.  In  belt-driven  generators 
the  pulleys  must  all  have  the  same  dimensions.  The  belts 
must  be  watched  to  see  that  they  do  not  slip.  These  two 


GENERATORS.  ^9 

|)oinls  must  be*  especially  observed  in  generators  driven 
horn  the  same  shaft.  The  speed  regulation  of  engines 
operating  direct  connected  alternators  in  parallel  is  dis- 
eussed  in  the'  following  section.  Water-wheels  have  an 
absolutely  uniform  angular  rotation,  and  are  the  best  prime 
movers  lor  parallel  running. 

Synchronism  ol  two  polyphase  generators  is  determined 
by  some  form  of  phase  Indicator,  The  commonest  arrange- 
ment consists  ol  two  transformers,  the  primaries  of  which 
are  connected  to  each  generator,  care  being  taken  that  the 
connections  are  made  to  similar  phases.  The1  secondaries 
are  connected  in  series  with  one  or  two  lamps  in  circuit. 
The  machine's  arc'  in  synchronism  when  the  lamps  cease 
to  glow.  Thev  may  then  hi-  thrown  in  parallel  by  the 
main  switches,  With  composite  Held  machines  the  com- 
mutators must  be  connected  bv  an  eciualizer  to  place  the' 
series  windings  in  multiple'.  The  connections  and  station 
Instruments  required  for  the  process  of  throwing  genera- 
tors in  parallel,  and  (operating  them  continuously,  as  used 
extensively  in  this  country,  are  shown  in  l''ig.  ^S. 

It  does  not  follow  that,  because'  one'  phase  of  a  polyphase 
circuit  is  synchronized,  the  other  phase's  are'  ready  for 
parallel  connection.  It  is  quite*  important  that  whc'n  a 
number  of  machines  are  first  installed  for  operating  in 
parallel,  the  connections  should  correspond  throughout  in 
all  the  machine's.  Tin*  e'ircuits  can  be  tc'stc'd  out,  lor 
proper  connection,  by  means  of  two  sets  of  phase'  lamps. 

In  the  diagram  (  I''ig.  39)  temporary  transformers  are' 
shown  connected  to  a  different  phase  of  the'  circuit  than 
that  in  which  are'  the  permanent  lamps.  Connection 
should  first  be  made'  with  the'  outside'  blades,  as  shown 
by  the  dolled  lines,  to  prove'  that  the  two  sets  of  lumps 


POLYPHASE   APPARATUS   AND    SYSTEMS. 


will  operate  together.  By  the  separate  connections  of 
the  temporary  transformers,  it  can  be  ascertained  if  the 
machines  are  properly  connected  to  the  synchronizing 


switches.     The  connections  are  correct  when  both  sets  of 
lamps  are  simultaneously  dark. 

Speed  Regulation  of  Engines.  —  Steam-engines  intended 
for  direct  connection  to  alternators,  which  supply  current 


GENERATORS. 


to  rotary  converters  or  synchronous  motors,  or  are  operated 
in  parallel,  should  be  designed  to  have  an  angular  rotation 
as  nearly  uniform  as  possible.  Otherwise  the  oscillations 
in  the  relative  motions  of  the  generators  or  of  the  genera- 
tors and  the  synchronous  apparatus  may  produce  an  exces- 
sive exchange  of  currents. 

The  amount  of  deviation  from  the  position  of  absolutely 
uniform  angular  speed  permissible  for  satisfactory  work 
depends  upon  a  number  of  conditions.  It  is  effected  by 
the  design  of  the  generator  and  rather  more  by  the  dif- 
ficulties of  operating  synchronous  apparatus.  For  the 
majority  of  cases  an  allowable  angular  variation  of  2j°  in 
phase  from  the  mean  will  produce  excellent  results.  This 
means  that  in  engines  direct  connected  to  alternators  of 
2  11  poles,  the  position  of  each  revolving  part  should  not 

To  Bus  Bars. 


Tempor  ary 


VVWWX                 c 

>s.r 

p  c 

SwiC 
p        c 

r     \wwv\A  —  y 

^  —  ' 

; 

To  Generator. 

Fig.  39. 


differ  more  than      -  in  circumference  from  the  position  it 
n 

would  have  at  absolutely  uniform  rotation.      Thus,  in  40 
polar  alternators,  the  maximum  deviation   from  the  posi- 


52    POLYPHASE  APPARATUS  AND  SYSTEMS. 

tion  of  uniform  rotation  for  parallel  operation  would  be 

2— 

—  or  |  degrees  of  circumference  for  each  unit. 

The  above  expresses  the  regulation  of  the  engine  as 
a  deviation  in  position  from  that  of  absolutely  uniform 
rotation  in  degrees  of  total  circumference  measured  for 
example  on  the  circumference  of  the  fly-wheel  or  revolving 
member  of  the  direct-connected  alternator. 

Single  cylinder  or  tandem  compound  engines  cannot 
as  a  rule  give  as  good  results  as  engines  whose  cranks 
are  quartering. 

The  difficulty  of  parallel  operation  of  alternators  is  not 
often  due  to  the  cyclic  irregularity  of  the  turning  moment 
of  the  prime  mover,  which,  by  changing  the  frequency  dur- 
ing each  revolution,  may  cause  hunting  of  synchronous  appa- 
ratus. Nor  has  the  design  of  the  alternators  any  material 
influence  upon  their  successful  parallel  running.  Mr.W.  L. 
R.  Emmet  was  the  first  to  demonstrate  that  successful  par- 
allel working  could  be  obtained  if  the  prime  movers  were 
fitted  with  anti-hunting  governors.  The  impulses  due  to 
changes  of  load  are  emphasized  by  freely  acting  governors 
which  tend  to  overrun  or  hunt.  The  cure  is  in  the  use  of 
dash-pots  which  will  retard  for  a  brief  period  any  motion  of 
the  valve  mechanism,  but  will  yield  to  continued  pressure. 

Methods  of  Driving  Generators.  —  The  mechanical  coup- 
ling of  a  generator  to  the  prime  mover  is  determined 
mainly  by  the  size  of  the  generator  and  the  type  and  speed 
of  the  prime  mover.  Polyphase  units  up  to  200  K.W.  are 
usually  belted,  unless  the  prime  mover  consists  of  a  water- 
wheel  of  high  speed,  or  special  conditions  favor  direct  con- 
nection to  an  engine.  The  mechanical  arrangement  of 


GENERATORS.  53 

the  generator  parts  is  shown  by  Fig.  40.  The  yoke  rests 
on,  and  is  sometimes  an  integral  part  of,  the  bedplate,  which 
also  supports  two  bearings.  The  pulley  is  overhung. 

The  method  of  belt-driving  larger  units  is  shown  in 
Fig.  41.  The  bedplate  is  extended,  and  carries  a  third  or 
outboard  bearing  which  partly  relieves  the  inner  bearing  of 
the  belt  strain  and  the  weight  of  the  pulley. 

Generators  designed  for  water-wheel  connection  are 
usually  provided  with  bedplate,  shaft  and  two  bearings. 
These  machines  are  self-contained  for  the  more  perfect 
alignment  of  the  bearings.  Fig.  42  illustrates  the  gene- 
ral arrangement  of  generators  of  500  K.W.  capacity  and 
above.  A  half-coupling  is  provided,  which  is  machined  to 
a  close  fit  with  the  other  half  furnished  with  the  water- 
wheel. 

Generators  for  direct  connection  to  engines  are  built 
without  bedplate,  shaft,  or  bearings.  The  yoke  rests  on  a 
thin  iron  soleplate  supported  by  a  suitable  foundation. 
The  engine  bearing  serves  also  for  the  inner  bearing  of 
the  generator.  The  outboard  bearing  rests  on  a  separate 
cap.  It  is  usually  furnished  with  the  engine,  and  is  of  a 
design  uniform  with  the  inner  bearing.  The  engine  shaft 
extended  carries  the  revolving  element  of  the  electrical 
unit  (Fig.  43). 

Polyphase  generators  above  500  K.W.  should  preferably 
be  direct-coupled  to  the  prime  mover.  The  method  of 
driving  large  generators  by  belts  or  ropes  necessitates  a 
large  extension  of  the  base,  and  a  heavy  pulley,  and  is 
mechanically  awkward.  This  method  of  driving  may  be 
used  in  exceptional  cases,  as,  for  instance,  in  connection 
with  a  wheel  plant  already  installed,  operating  under  a 
very  low  head  at  a  low  speed.  The  increased  cost  of 


54  POLYPHASE   APPARATUS    AND    SYSTEMS. 


GENERATORS. 


55 


56  POLYPHASE   APPARATUS    AND   SYSTEMS. 

extended  shaft,  outboard  bearing,  and  pulley  will  go  far 
towards  offsetting  the  increased  cost  of  a  slower  speed 
generator,  for  direct  connection,  which  does  not  require 
these  parts. 

Polyphase  generators  are  direct-connected  to  water- 
wheels,  either  by  a  vertical  or  by  a  horizontal  shaft.  Very 
few  generators  in  this  country  run  from  vertical  turbines. 
The  notable  exceptions  are  the  large  generators  at  Niag- 
ara Falls,  and  those  in  the  station  of  the  Portland  General 
Electric  Company  at  Oregon  City,  Oregon.  The  advan- 
tages of  the  vertical  connection  lie  in  the  saving  of  floor- 
space,  requiring  a  smaller  power-house,  and  in  more  respon- 
sive wheel  regulation.  The  shafting  is  out  of  sight,  the 
revolving  parts  reduced  to  a  minimum,  and  the  effect,  as  a 
whole,  most  pleasing.  The  disadvantages  are,  the  increased 
cost,  and  a  possible  mechanical  difficulty  in  supporting  the 
vertical  shaft,  weighted  with  the  revolving  electrical  and 
hydraulic  parts.  The  European  practice  is  to  almost  ex- 
clusively employ  the  vertical  generator  in  connection  with 
vertical  water-wheels. 

Horizontal  generators,  for  direct  coupling  to  turbines,  are 
usually  so  constructed  that  the  lower  frame  forms  one  part 
of,  or  rests  on,  a  base,  which  also  supports  the  two  standards. 
Sometimes,  as  shown  in  Fig.  22,  an  extension  and  third 
bearing  is  used,  the  water-wheel,  properly  housed,  taking 
the  place  of  the  pulley.  Such  an  arrangement  is  pecu- 
liarly adapted  for  use  with  impact  wheels.  This  construc- 
tion is  used  in  the  power  plants  of  the  Big  Cottonwood 
Electric  Company,  the  Pioneer  Electric  Company,  Ogden, 
Utah,  and  the  Southern  California  Power  Company,  Red- 
lands,  Cal.  Perfect  and  permanent  alignment  of  bearings 
is  obtained  by  this  construction. 


GENERATORS. 


57 


A  typical  Westinghouse  two-phase  generator  of  the  re- 
volving   armature    type,   for    direct    connection    to  water- 


1 


Fig-.  44. 

wheels,  is  illustrated  in  Fig.  44.  Where  engines  are  direct- 
connected  to  polyphase  generators,  it  is  customary  for  the 
electrical  manufacturers  to  furnish  the  machine  without 


58         POLYPHASE   APPARATUS   AND   SYSTEMS. 

shaft,  base,  or  bearings.  For  the  same  speed,  therefore, 
engine-driven  generators  are  cheaper  than  those  driven  by 
water-wheels.  It  must  not  be  forgotten,  however,  that 
engine  speeds  are  limited  by  a  number  of  conditions,  while 


-  45. 


water-wheels  are  practically  limited  in  speed  only  by  the 
head  obtainable. 

Fig.  45  illustrates  a  three-phase  generator  of  1,200  H.P. 
capacity,  direct-connected  to  an  engine  running  at  94 
R.P.M. 

This    generator  is  direct-coupled    to  a   Corliss  type    of 


GENERATORS.  59 

engine  of  1,300  indicated  H.P.,  running  at  94  revolutions. 
It  has  32  poles,  and  gives  a  current  at  5,000  volts  and  a 
frequency  of  25  cycles.  The  armature  windings  consist  of 
96  coils,  three  for  each  pole,  or  two  slots  per  phase  per  pole. 
The  windings  are  Y  connected.  The  field  coils  are  flat  strip- 
copper,  i  in.  by  -^  in.,  wound  on  edge,  and  insulated  by 
intervening  layers  of  paper.  As  the  exciting  current  has  a 
pressure  of  not  greater  than  1 20  volts,  the  potential  at  the 
terminals  of  each  field  spool  is  about  four  volts.  The  effi- 
ciency of  the  generator  is  95 1-  per  cent  at  full  load,  94^  per 
cent  at  three-quarter  load,  92^  per  cent  at  half  load,  and 
87  per  cent  at  quarter  load.  The  regulation  on  non-induc- 
tive load  is  6  per  cent,  and  the  exciting  current  about  1 20 
amperes. 

Engine-driven  generators  are  sometimes  constructed 
with  their  field  magnets  built  out  as  integral  parts  of  the 
engine  fly-wheel,  they  with  their  windings  being  fastened 
to  the  external  periphery.  These  machines  are  known  as 
Fly-wheel  Alternators.  The  largest  machines  of  this  type 
yet  constructed  were  designed  by  the  Westinghouse  Co. 
for  the  Manhattan  Railway  Co.  of  New  York.  They  have 
a  nominal  rating  of  5000  K.W.  with  50^  overload  capacity, 
height  42  feet,  diameter  of  revolving  part  about  32  feet, 
weight  185  tons.  The  revolving  field  construction  con- 
sists of  a  steel  hub  to  which  are  built  a  dovetailed  annular 
roll  of  steel  web  plates  which  form  the  driving  spider. 
The  field  poles  and  rim  are  built  up  with  overlapped  plates 
of  thin  sheet  steel,  the  length  of  each  plate  being  equal  to 
2  poles.  The  plates  are  dovetailed  into  the  driving  spider, 
and  the  rim  and  poles  with  their  steel  end  plates  are  sepa- 
rately bolted  together.  In  the  body  of  the  field  poles  at 
intervals  of  about  3  inches  ventilated  spaces  or  ducts  are 


60    POLYPHASE  APPARATUS  AND  SYSTEMS. 

provided.  These  spaces  extend  inward  to  the  laminated 
steel  rim  of  the  fly-wheel  to  large  openings  in  the  cast- 
iron  driving  rim.  The  ventilating  ducts  in  the  revolving 
field  register  with  corresponding  ducts  in  the  external 
stationary  armature.  The  field-pole  tips  are  beveled  at 
the  edges  to  produce  a  magnetic  distribution  of  such  a 
form  that  with  a  star  type  of  3-phase  winding  the  E.J\I.F. 
wave  will  be  practically  a  sine  wave  at  no  load.  The 
speed  is  75  R.P.M.,  poles  40,  frequency  25.  The  field 
requires  225  amperes  at  200  volts  when  the  machine  is 
delivering  its  rated  current  at  1 1,000  volts  on  a  non- 
inductive  load.  About  15%  more  current  is  required 
when  the  armature  delivers  its  full  rated  output  at  normal 
voltage  to  a  circuit  having  a  power  factor  of  90%.  The 
regulation  is  such  that  if  load  of  263  amperes  per  phase, 
11,000  volts,  and  with  \oof/c  power  factor  be  thrown  off, 
the  potential  will  rise  not  more  than  6r/(  ;  field  excitation 
and  speed  remaining  constant.  It  is  calculated  on  non-in- 
ductive load,  that  the  efficiency  will  range  from  90%  at 
quarter  load  to  96%%  at  full  load,  mechanical  friction  not 
being  included. 


Conditions  Affecting  Cost.  —  From  what  has  preceded, 
it  will  be  easily  understood  that  the  first  factor  in  deter- 
mining the  cost  of  a  polyphase  generator  of  a  given  capacity 
and  conditions  of  operation  is  its  speed.  The  initial  vol- 
tage is  another  factor,  and  likewise  the  efficiency  and  the 
regulation.  A  generator  wound  for  a  high  voltage  may, 
under  certain  conditions  of  proportion  and  design,  cost 
more  to  construct  than  a  low  voltage  machine  of  same 
characteristics,  together  with  its  step  up  transformers.  A 


GENERATORS. 


6l 


generator  of  high  efficiency  can  be  built  at  a  reasonable 
cost,  but  at  the  expense  of  some  regulation.  The  same 
generator  may  have  better  regulation  at  the  sacrifice  of 
efficiency,  and  cost  no  more.  To  obtain  both  these  con- 


4UU 

300 

3S 

Q," 

ec 

£• 

200 

inn 

1 

7^ 

1 

/ 

/ 

/ 

1 

1 

/ 

I 

/ 

/ 

/ 

f 

/ 

/ 

/ 

/ 

^ 

/ 

10 


50 


20  30  40 

Per  Cent  Reduction  in  Cost 

Fig-.  46. 

stants,  in    an  eminent    degree,  requires    a    liberal   use  of 
copper  and  iron,  and  results  in  an  expensive  machine. 

The  frequency  of  the  current  for  which  a  generator  is 
designed  is  another  determining  factor  in  the  cost.  With 
a  given  speed,  changing  the  frequency  alters  the  number 


62    POLYPHASE  APPARATUS  AND  SYSTEMS. 

of  poles ;  correspondingly,  a  reduction  in  the  number  of 
poles  cheapens  a  generator.  Less  exciting  copper  is 
needed ;  for,  while  the  polar  cross-section  is  unchanged, 
the  average  length  of  turn  is  less.  The  number  of  opera- 
tions in  manufacture  and  handling  are  also  considerably 
reduced.  The  effect  of  change  of  frequency  on  the  cost  is 
most  noticeable  in  very  slow-speed  direct-connected  units. 
Take  the  case  of  a  133  cycle  generator,  direct-connected 
to  an  engine,  running  at  approximately  300  revolutions  per 
minute.  To  give  the  proper  frequency,  it  must  have  52 
poles.  By  reducing  the  frequency  to  40  cycles,  16  poles 
are  needed.  It  must  not  be  forgotten,  however,  that  low- 
ering the  frequency  of  any  piece  of  alternating  apparatus 
necessitates  an  increase  in  the  iron  of  the  magnetic  cir- 
cuit. Iron,  however,  is  cheap  as  compared  with  copper 
and  price  of  labor.  Of  course,  the  proportionate  saving 
is  not  so  noticeable  in  high  speeds,  nor  when  the  genera- 
tors are  belt-driven,  or  provided  with  parts  that  remain  the 
same  irrespective  of  the  frequency. 

Fig.  46  shows,  in  an  approximate  degree,  the  relative 
reduction  in  cost  with  increasing  speed.  In  using  this 
curve  for  comparison  of  costs,  it  must  be  kept  in  mind  that 
it  is  approximately  correct  only,  and  applies  to  generators 
of  the  same  type,  frequency,  general  constants,  and  condi- 
tions of  operation. 


INDUCTION    MOTORS.  63 


CHAPTER    IV. 
INDUCTION    MOTOR$. 

Principles  of  Operation The  induction  motor  can  be 

compared  to  a  direct-current  shunt  motor,  the  essential 
difference  being  that  the  armature  or  working  current  of 
the  shunt  motor  is  led  into  it  by  brushes,  while  the  work- 
ing current  of  the  induction  motor  is  an  induced  or  trans- 
former current.  The  windings  of  the  induction  motor, 
connected  to  the  supplying  circuit,  besides  carrying  the 
exciting  current,  have  the  additional  function  of  supply- 
ing the  transformer  current.  The  induction  motor  is  thus 
seen  to  combine  the  principles  of  operation  of  both  a 
motor  and  a  transformer.  Rotation  may  be  considered  as 
being  produced  by  the  revolving  member  following  a  shift- 
ing magnetic  field  which  is  the  resultant  of  two  or  more 
alternating  fields  differing  in  phase.  The  explanation  of 
the  working  of  the  induction  motor  by  reference  to  the 
rotating  magnetic  field  alone,  however,  is  apt  to  mislead 
and  to  hide  its  true  functions. 

The  two  elements  of  an  induction  motor  are  preferably 
designated  as  primary  and  secondary,  and  sometimes  as 
field  and  armature.  Either  may  be  indifferently  the  rotor 
or  stator. 

When  running  without  load,  the  rotor  speed  is  very 
closely  that  of  the  rotating  field,  and  there  is  a  very 
small  current  induced  in  the  secondary.  The  magnetic 


64    POLYPHASE  APPARATUS  AND  SYSTEMS. 

pull  of  this  current  on  the  field  produces  a  feeble  torque. 
The  current,  taken  by  the  primary  member,  or  field,  is 
then  composed  of  the  magnetizing  current  and  that  re- 
quired for  overcoming  magnetic  and  mechanical  friction. 
As  the  power  factor  is  low  at  light  loads,  being  not  more 
than  1 5  or  20  per  cent  in  most  commercial  motors,  the 
energy  supplied  is  not  much  greater  than  that  consumed 
by  a  shunt  motor  of  the  same  capacity. 

When  running  under  load,  the  speed  of  the  revolving 
element  falls  away  from  that  of  synchronism,  and  the 
E.M.F.  and  working  current  induced  by  the  relative 
cutting  of  the  lines  of  force,  increase  with  the  difference 
in  speeds.  The  pull  of  this  increased  current  on  the  field 
produces  a  powerful  torque.  The  departure  from  the 
speed  of  synchronism  is  called  the  "slip,"  and,  within  cer- 
tain limits,  is  proportional  to  the  total  secondary  resistance. 

To  insure  high  efficiency  and  good  regulation,  the  resist- 
ance of  the  shunt  motor  armature  must  be  kept  as  low  as 
practicable.  For  the  same  reason,  the  windings  of  the 
secondary  of  the  induction  motor  should  have  a  low  re- 
sistance. 

Methods  of  Starting  Motors On  connecting  an  induc- 
tion motor  to  its  supplying  circuit,  there  is  an  excessive 
rush  of  current,  which  can  be  prevented  only  by  the  use  of 
some  device  external  to  the  motor  windings  proper.  There 
are  a  number  of  such  arrangements  for  reducing  the  start- 
ing current  of  motors.  Two  of  these  are  extensively  used 
in  this  country,  and  will  be  described.  The  others  are  of 
less  commercial  importance. 

The  first,  and  probably  most  common  device,  consists 
essentially  of  a  variable  resistance,  which  can  be  cut  in 
or  out  of  circuit  with  the  secondary  winding.  When  the 


INDUCTION    MOTORS.  65 

secondary  element  is  the  rotor,  this  resistance  often 
occupies  a  space  within  the  armature  spider.  It  may 
be  of  copper  strips,  or  —  as  is  usually  the  case  —  of  iron 
cast  into  a  compact  .grid  form,  having  a  number  of  con- 
tact points.  The  whole  of  this  resistance  is  in  series 
with  the  secondary  winding  at  starting.  As  the  motor 
attains  speed,  a  circular  short-circuiting  switcn,  mounted 
in  a  ring  encircling  the  shaft,  is  pushed  centrally  by  a 
lever,  thus  cutting  out  the  resistance  in  as  many  succes- 
sive steps  as  there  are  contact  points.  Motors  provided 
with  this  starting  device  are  usually  designed  to  start  with 
a  torque  ranging  from  75  to  150  per  cent  of  full-load 
torque.  This  motor  has  the  desirable  characteristic  that 
the  current  is  very  nearly  proportional  to  the  torque  from 
starting  to  full-load  speed. 

In  some  motors  of  European  make,  an  external  rheostat 
is  used  to  cut  down  the  induced  current.  When  the  sec- 
ondary revolves,  collector  rings  are  required  to  convey  the 
induced  current  to  the  rheostat.  When  the  primary  is 
the  revolving  element,  collector  rings  are  also  needed  to 
supply  the  main  current  to  the  motor. 

A  water  rheostat  is  sometimes  employed  abroad,  by 
means  of  which  the  induced  current  is  varied,  its  strength 
varying  with  the  depth  to  which  the  plates  are  immersed. 
With  this  device,  the  current  taken  by  the  motor  is  closely 
proportional  to  the  torque,  from  starting  to  full-load  speed. 

The  second  method  of  starting  induction  motors  con- 
sists in  reducing  the  impressed  volts  by  the  use  of  some 
form  of  reactance  or  of  compensator  coils,  or  of  resistance 
in  the  main  circuit.  A  compensator  is  the  most  efficient 
means  of  cutting  down  the  voltage,  and  the  most  gen- 
erally employed,  one  coil  being  required  for  each  phase. 


66 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


The  connections  of  a  Westinghouse  two-phase  motor  and 
starting  device  are  shown  in  Fig.  47.  The  starter  consists 
of  two  coils,  sometimes  called  auto-converters  or  com- 
pensators, one  in  each  phase.  Each  coil  is  arranged  to 
give  a  number  of  different  starting-voltages  to  suit  differ- 
ent conditions  of  operation.  Fig.  48  shows  the  connec- 
tions of  this  starting  device  in  detail.  The  switch  is 
down  for  starting  the  motor,  and  after  speed  has  been 
reached,  is  thrown  up  to  its  running  position,  thereby  cut- 


Phase  A 


Phase  B 


Fig-.  47. 

ting  out  the  compensator.  By  this  arrangement  the  motor 
can  be  started  at  a  distance.  Connections  can  be  made 
from  I  to  either  2,  3,  or  4,  giving  three  different  starting 
electromotive  forces  and  starting  torques.  The  maximum 
E.M.F.  and  torque  are  obtained  by  connecting  I  and  4  ; 
for  minimum  /f.J/./**.,  i  and  2  are  connected.  The  con- 
nections of  a  starting  compensator  for  a  three-phase  motor, 
as  made  by  the  General  Electric  Company,  is  shown  in 
Fig.  49.  As  in  the  two-phase  starter,  there  is  a  coil  in 
each  phase,  with  a  number  of  taps.  These  compensator 


INDUCTION    MOTORS. 


67 


starters,  for  use  with  motors  of  15  H.P.  and  under,  have 
three  taps  with  voltages  40  per  cent,  60  per  cent,  and 
80  per  cent  of  running  full-load  voltage.  Compensators 
for  motors  above  15  H.P.  have  four  taps,  giving  voltages 
40  per  cent,  58  per  cent,  70  per  cent,  and  85  per  cent  of 
running  full-load  voltage. 

Induction  motors  which  are  put  in  operation  by  the 
first  method,  may  be  designated  as  the  variable  resistance- 
in-armature  type.  They  frequently  have  a  higher  self- 
induction,  and  require  more  copper  and  less  iron. 


I  Running  Position 


Off  Position 


I  Starting  Position 


Fig.  48. 


secondary  winding  is  definite  and  polar.  Motors  which 
are  used  with  the  compensator  starter  may  be  designated 
as  the  compensator  or  short-circuited-armature  type.  They 
are  proportioned  so  that  the  primary  and  secondary  have 
a  low  self-induction.  They  contain  a  minimum  amount  of 
copper  and  a  considerable  amount  of  iron  in  the  magnetic 
circuit,  and  a  short  air-gap.  Their  distinctive  feature  is 
the  short-circuited  armature,  which  is  usually  of  the  squir- 
rel-cage construction. 

In  starting  an  induction  motor  with  variable  secondary 
resistance,   precaution   must  be  taken  that  the  resistance 


68         POLYPHASE  APPARATUS   AND   SYSTEMS. 


INDUCTION    MOTORS.  69 

is  all  in,  otherwise  the  flow  of  current  may  overheat  the 
motor  or  overload  the  lines.  The  armature  lever  should 
be  pulled  out  as  far  as  it  will  go  ;  then  the  line  switch  may 
be  closed,  and,  finally,  the  short-circuiting  switch  may  be 
slowly  closed.  The  motor  should  be  handled  at  starting 
to  reach  full  speed  in  about  fifteen  seconds.  As  the 
secondary  resistance  is  of  a  capacity  only  to  start  the 
motor,  it  never  should  be  left  in  circuit  or  used  to  regu- 
late the  speed  of  the  motor.  The  motor  is  shut  down  by 
reversing  the  operations  of  starting. 

As  the  drop  in  a  good  transformer  on  a  lighting  load 
is  within  3  per  cent,  and  on  an  inductive  load,  as  motors, 
seldom  less  than  5  per  cent,  it  is  advisable  to  always  use 
separate  transformers  for  lights  and  for  motors.  The 
exception  to  this  rule  is  in  a  secondary  system  of  distribu- 
tion, where  the  motor  load  is  a  proportionately  small  part 
of  the  entire  load. 

Induction  motors  are  sometimes  started  by  being  con- 
nected directly  to  the  supplying  circuit  without  the  use  of 
any  form  of  starting  device.  Such  a  motor  will,  of  course, 
take  a  large  starting  current.  This  can  be  kept  down  by 
making  the  resistance  of  the  armature  conductors  rather 
high,  and  by  confining  the  motor  to  work  requiring  a  small 
starting  torque.  A  motor  started  in  this  way  should  not  be 
used  on  circuits  where  the  effect  of  a  large  starting  current 
on  the  potential  regulation  of  the  system  is  of  importance. 

A  three-phase  induction  motor  is  reversed  by  changing- 
any  two  of  the  leads,  and  a  two-phase  by  changing  the  two 
leads  of  either  phase. 

Construction  of  Primary  and  Secondary The  simple 

and  substantial  construction  of  the  induction  motor  is  one 
of  its  chief  advantages,  resulting  in  a  minimum  cost  of 


70          POLYPHASE   APPARATUS   AND    SYSTEMS. 


Fig.  50. 


maintenance  and  attendance.  While  either  element  may 
be  the  rotor,  by  far  the  larger  number  of  commercial 
motors  are  now  constructed  with  a  fixed  primary  and  with 
a  rotor  secondary. 


INDUCTION    MOTORS.  71 

The  fixed  primary  may  be  likened  to  an  inverted  arma- 
ture. It  is  built  up  of  slotted  laminations  mounted  on  a 
cast-iron  spider.  The  coils  are  imbedded  in  the  slots. 
Fig.  50  illustrates  a  Westinghouse  primary  or  field  ready 


Fig.  51. 


to  receive  its  conductors.  These  stationary  windings  are 
usually  protected  from  mechanical  injury  by  end  shields, 
which  frequently  support  the  bearings.  The  Westing- 
house  Company  employ  this  form  of  construction  in  even 


72         POLYPHASE   APPARATUS    AND    SYSTEMS. 

the  largest  sizes,  as  illustrated  in  Fig.  51,  which  represents 
a  500  H.P.  motor. 

This  motor  is  wound  for  three-phase  current  at  60  cycles 
and  400  volts.  It  has  36  poles,  running,  therefore,  at  200 
R.P.lf.  The  secondary  has  a  squirrel-cage  winding,  bar 
wound  as  is  the  primary.  The  starting  torque  is  two  and 
one-fourth  times  the  full-load  rated  torque.  The  drop  in 
speed  from  no  to  full  load  is  4  per  cent.  The  power  factor 
at  full  load  is  given  as  93  per  cent.  The  dimensions  are : 


Fig-.  52. 

Height,  10  feet  3  inches  ;  floor  space  occupied,  9  feet  6 
inches  by  3  feet  6  inches  ;  diameter  at  air-gap,  7  feet. 
The  total  weight  is  42,000  pounds.  This  motor  is  direct- 
coupled  to  a  line  shaft,  driving  a  mill  in  Mexico. 

The  rotor  armature  of  the  standard  form  of  motor  has 
a  laminated  slotted  structure  similar  to  the  primary.  In 
motors  of  the  variable  resistance  type,  the  secondary  has 
a  definite  series  of  coil  windings,  corresponding  to  the  polar 
windings  of  the  primary.  Motors  of  the  short-circuited 
type  are  generally  wound  with  copper  bars  laid  in  the  slots 


INDUCTION    MOTORS. 


73 


and  connected  at  both  ends  by  short-circuiting  metal  rings. 
Secondaries  of  this  construction  are  termed  squirrel-cage 
armatures.  Fig.  52  shows  an  armature  wound  in  the 
manner  described  and  illustrative  of  this  type. 

In  the  Stanley  Company's  motor  (Fig.  53)  the  field  is 
stationary.     There  are,  in  reality,  two  fields  and  two  arma- 


Fig.  53. 

tures.  The  secondary  windings  are  connected  so  that  the 
wire  lying  under  the  field  poles  on  one  armature  is  in 
series  with  the  wire  lying  between  the  poles  on  the  other. 
The  field  coils  are  staggered,  each  half  alternately  playing 
the  part  of  a  motor  and  transformer. 

Starting  Torque  and  Current.  —  At  normal  voltage  cer- 


74 


POLYPHASE   APPARATUS    AND    SYSTEMS. 


tain  types  of  motors  possessing  a  moderate  secondary  re- 
sistance,—  as,  foMnstance,  a  motor  of  the  variable  resist- 
ance type,  with  the  resistance  cut  out,  —  will  have  a  small 
starting  torque  due  to  the  reaction  of  the  excessive  induced 
secondary  current,  on  the  primary.  The  starting  current 
consumed  by  the  motor  will  likewise  be  excessive.  At 


Standstill 


Armature  Slip 
Fig.  54. 


Synchronism 


nearly  synchronous  speed  such  a  motor  will  have  a  pow- 
erful torque.  By  increasing  the  secondary  resistance,  the 
starting  torque  is  raised  until  a  critical  resistance  is  reached, 
beyond  \vhich  point  the  starting  torque  decreases. 

The  starting  torque  of  an  induction  motor  is  also  de- 
pendent upon  the  potential  applied  at  its  terminals.  The 
starting  current  is  reduced  by  lowering  the  voltage,  but  at 
the  sacrifice  of  the  torque  at  starting,  which  varies  as  the 
square  of  the  volts. 


INDUCTION   MOTORS.  75 

An  inspection  of  the  curves  in  Fig.  54  will  show  how 
the  starting  torque  is  influenced  by  varying  the  secondary 
resistance.  The  secondary  winding  of  the  motor  is 
assumed  to  have  a  fixed  resistance  of  .02  ohms.  At  start- 
ing, a  variable  resistance  is  connected  in  series,  making  a 
total  of  .18  ohms.  The  corresponding  torque  is  about  25 
pounds,  or  150  per  cent  of  full-load  torque.  When  the 
motor  reaches  about  50  per  cent  of  synchronism,  part  of 


20       40      GO       80     100    120     140    100     180     200     220    240     2GU    230    300     320    340    36( 
Horse  Power  Output 

Figf.  55. 

the  resistance  is  cut  out,  making  the  total  .045  ohms.  The 
torque  now  increases  until  about  85  per  cent  of  synchro- 
nous speed  is  reached,  when  it  begins  to  drop.  At  this 
point  the  remaining  resistance  is  short-circuited,  leaving 
only  the  resistance  of  the  secondary.  The  torque,  due  to 
this  resistance,  .02  ohms,  reaches  its  maximum  at  about 
90  per  cent  of  synchronism.  The  starting  torque,  with  a 
secondary  resistance  of  .02  ohms,  is  about  seven  pounds. 
The  starting  torque,  due  to  a  resistance  of  .751  ohms,  is  less 


76    POLYPHASE  APPARATUS  AND  SYSTEMS. 

than  when  the  total  secondary  resistance  is  .18  ohms,  be- 
ing only  1 6  pounds.  The  current  in  the  primary  of  such 
a  motor  at  all  speeds  will  be  nearly  proportional  to  the 
torque  developed.  At  the  moment  of  cutting  out  the  suc- 
cessive resistances,  the  current  will  momentarily  increase  in 
strength.  It  can  be  readily  seen  that,  by  using  a  sufficient 
number  of  resistance  steps,  the  motor  could  be  brought  up 
to  speed  with  uniform  torque  and  current.  When  the  motor 
is  taxed  beyond  its  capacity,  its  torque  and  speed  rapidly 
diminish  and  a  large  current  will  flow.  This  break-down 
point  is  determined  by  the  design  of  the  motor,  and  is 
fixed  at  from  50  to  100  per  cent  greater  than  the  rated 
load.  The  working  point  of  such  a  motor  is  on  the  de- 
scending portion  of  the  power  curve,  at  about  two-thirds  of 
the  maximum  output.  Curves  of  torque  and  amperes  input 
at  all  loads,  of  a  175  H.P.  motor,  are  given  in  Fig.  55. 

The  magnetizing  current  which  is  characteristic  of  most 
alternating-current  apparatus,  such  as  transformers,  induc- 
tion motors,  etc.,  has  the  effect  of  increasing  the  full-load 
current  and  putting  a  greater  demand  on  transformers,  line, 
and  generators.  The  total  current  is  greater  than  that 
actually  required  in  supplying  the  losses  and  doing  the 
work  of  the  motor.  The  ratio  of  this  working  or  energy 
current  to  the  total  current  gives  the  power  factor. 

As  the  starting  current  of  the  motor,  with  short-circuited 
armature,  is  reduced  by  lowering  the  voltage,  it  follows 
that,  for  the  same  starting  torque  as  that  developed  by  the 
variable  resistance  type,  the  current  will  be  considerably 
greater. 

The  line  starting  current  and  the  torque  of  some  makes 
of  motors  with  short-circuited  armatures,  expressed  in  per- 
centages of  full  load,  are  about  as  follows  :  — 


INDUCTION    MOTORS. 


77 


The  local  current  between  the  compensator  and  motor 
will  be  greater  than  the  line  starting  current,  as  its  poten- 


E.M.F. 

40% 
60% 
80% 

100% 


STARTING  CURRENT. 

112% 


250% 
45°% 
700% 


STARTING  TORQUE. 

32% 

72% 
128% 
200% 


tial  is  lower.     The  action  of  the  compensator  is  similar  to 
that  of  a  transformer. 

By  increasing  the  resistance  of   the  armature  of  these 


Double  Thro_w-Switch 
Running  Side    t~° 


Generator 


Motor 


Double  Throw-Switch 
Running  Side 


Motor 


Figs.  56  and  57. 

motors,  the  starting  current  for  the  same  torque  is  de- 
creased ;  but  the  result  is  a  loss  of  efficiency,  which  may 
be  as  great  as  2  or  3  per  cent. 

Where  transformers  are  used  for  individual  motors,  or 
where   several  motors  are  located  close  to,  and   operated 


78  POLYPHASE   APPARATUS  AND    SYSTEMS. 

from,  a  bank  of  transformers,  it  is  sometimes  practical  to 
bring  out  taps  from  the  secondary  winding,  and  use  a 
double  throw  motor  switch,  thereby  making  provision  for 
starting  the  motor  at  low  voltage,  and  saving  the  cost  of 
a  compensator.  The  connections  of  such  an  arrangement 
are  shown  in  Fig.  56  and  Fig.  57. 

Motors  of  the  variable-resistance  type  have  a  special 
range  of  usefulness  when  operated  from  circuits  requiring 
good  regulation  such  as  is  demanded  in  central  station 
work.  They  are  desirable  for  service  where  the  motors 
are  apt  to  run  considerably  underloaded. 

The  short-circuited  type  is  to  be  recommended  for 
power  circuits,  and  when  the  motors  must  be  started  from 
a  distance  and  simplicity  of  operation  is  of  moment.  It  is 
adapted  for  service,  calling  for  low  starting  efforts  and  con- 
stant full  load,  and  is  especially  advantageous  when  the 
motors  are  apt  to  run  overloaded,  or  on  circuits  of  varying 
voltage.  It  is  not  adapted  for  lighting  circuits,  where  good 
regulation  is  important,  unless  the  current  at  starting  is 
small  when  compared  with  the  capacity  of  feeders  and  gen- 
erators. Fig.  58  represents  a  75  H.P.,  three-phase  motor 
of  this  type,  made  by  the  General  Electric  Company. 

Speed  Regulation.  —  Absolutely  synchronous  speed  is 
never  attained  in  an  induction  motor,  as  some  slip  is 
required  to  furnish  the  current  consumed  by  the  light- 
load  losses.  Under  increasing  load,  the  speed  will  fall 
away  from  synchronism  until  the  break-down  point  is 
reached,  and  if  the  motor  is  not  relieved  of  its  load,  it  will 
come  to  a  standstill.  The  current  will  then  be  at  its  maxi- 
mum. The  fall  in  speed  from  that  at  light  load  to  that 
at  normal  rated  load  will  vary  in  some  types  of  induction 
motors  from  i£  per  cent,  as  in  motors  of  100  H.P.,  to 


INDUCTION    MOTORS. 


79 


3  per  cent,  as  in  smaller  motors.  Motors  constructed 
with  high  and  fixed  secondary  resistances  may  drop  in 
speed  as  much  as  9  per  cent. 

The  complaint  has  been  made  against  the  induction 
motor  that  it  is  an  inflexible  piece  of  apparatus  in  respect 
to  regulation  of  speed.  It  is  quite  true  that  wide  varia- 
tions of  speed  are  obtained  in  modern  motors  only  at  the 
expense  of  efficiency  and  increased  cost  of  construction. 


Fig.  58. 

There  are  a  number  of  methods  of  obtaining  a  varia- 
tion of  speed  in  an  induction  motor. 

The  method  now  most  employed  is  that  by  rheostatic 
control.  A  resistance  is  intercalated  in  the  secondary 
circuit,  which  can  be  varied  by  short  successive  steps. 
The  range  of  speed  usually  demanded  of  a  variable  speed 
induction  motor  does  not  permit  the  use  of  the  small 


80        POLYPHASE    APPARATUS    AND    SYSTEMS. 

resistance,  such  as  is  used  in  starting  in  some  designs  of 
motor,  and  which  is  located  within  the  rotor  armature. 
An  external  rheostat  is  required,  of  sufficient  size  to  dissi- 
pate a  considerable  amount  of  energy.  Fig.  59  shows  the 
connections  of  a  three-phase  motor  and  of  a  rheostatic  con- 
troller for  variable  speeds.  Collector  rings,  as  shown,  must 
be  added  to  motors  having  revolving  secondaries  fcr  elec- 
trically connecting  the  windings  and  external  resistarce. 
The  main  line  is  shown  as  passing  through  the  control- 


Controller 


Fig.  59. 

ler.  By  this  arrangement  the  circuit  is  closed  simulta- 
neously with  the  commencement  of  the  operation  of  cutting 
out  the  resistance.  In  large  motors  the  controller  is  sepa- 
rate from  the  resistance,  being  connected  to  it  by  cables. 
It  is  in  appearance  similar  to  the  well-known  street-car 
controller,  and,  like  it,  is  reversible. 

When  two  motors  are  employed  together  in  the  same 
class  of  service,  as  for  instance  in  polyphase  railway  work, 
the  speed  may  be  reduced  one-half  and  less  by  the  method 
of  concatenated  or  tandem  control.  This  method  consists 


INDUCTION    MOTORS. 


8l 


in  feeding  the  secondary  of  one  motor  which  is  connected 
to  the  supply  circuit  direct  to  the  primary  of  the  second 
motor.  The  motors  are  not  necessarily  of  the  same  con- 
struction, but  must  be  provided  with  collector  rings  and 
brushes.  The  secondary  current  of  the  first  motor  fur- 
nishes power  to  the  second  motor  instead  of  being  dissi- 
pated in  a  rheostat,  thus  directly  increasing  the  efficiency 


Fig.    6O. 

of  the  first  motor.  The  secondary  current  of  the  second 
motor  is  regulated  by  a  resistance  in  circuit.  A  speed 
lower  than  half  is  obtained  by  increasing  this  resistance. 
A  higher  speed  is  obtained  by  connecting  the  motors  in 
parallel  on  the  supply  circuit  with  their  secondaries  feeding 
into  regulating  resistances.  The  tandem  method  of  con- 
trol lowers  the  power  factor  of  the  f:rst  motor  and,  thereby, 


82    POLYPHASE  APPARATUS  AND  SYSTEMS. 

its  torque.  The  speed-controlling  mechanism  is  quite 
complicated. 

A  water  rheostat  for  varying  the  secondary  resistance, 
and,  correspondingly,  the  speed,  of  the  motor,  is  used  by 
Ganz  &  Co.  in  a  three-phase  railway  plant  in  Northern 
Italy.  The  general  form  of  this  rheostat  is  shown  in 
Fig.  60.  Compressed  air  is  admitted  below  the  bottom 
of  the  tank,  raising  it,  and  thus  increasing  the  depth  of 
immersion  of  the  metal  plates. 

The  speed  of  an  induction  motor  can  also  be  controlled 
by  changing  the  impressed  volts  at  the  motor.  This 
method  requires  the  use  of  an  external  reactance  or  a 
compensator,  and  a  motor  possessing  a  high  fixed  armature 
resistance. 

The  controller  and  compensator  are  usually  separate. 
By  a  sufficient  number  of  taps  in  the  latter,  connected  by 
cables  to  the  controller,  a  graduated  variation  of  the  im- 
pressed volts  is  obtained,  and  a  corresponding  variation  in 
speed. 

Another  method  of  controlling  the  speed  is  by  chan- 
ging the  number  of  poles.  When  a  variety  of  speeds  is  re- 
quired, this  method  is  complicated,  requiring,  in  addition  to 
a  compensator,  an  elaborate  switching  device.  It  is  objec- 
tionable also,  as  the  motor  can  only  run  at  full,  one-half, 
and  one-quarter  speed,  and  at  no  intermediate  speeds. 
This  method  has  been  successfully  employed  in  cases 
where  half  speed  and  half  full-load  torque  are  required. 

An  investigation  of  the  relative  efficiencies  and  power 
factors  of  induction  motors  of  10  H.P.  output,  equipped 
with  the  rheostatic  and  with  the  potential,  variable  speed- 
controlling  devices,  gives  the  approximate  results  shown 
in  the  following  table  : 


INDUCTION   MOTORS. 


SPEED. 

METHOD  OF 
CONTROL. 

EFFICIENCY. 

P.   F. 

AP.  EF. 

Full  

(    Rheostatic 

S3 

86 

72 

I    Potential 

83 

86 

72 

Half  

{    Rheostatic 

41-5 

86 

36 

I    Potential 

36 

57 

20.5 

Quarter  . 

j    Rheostatic 
I    Potential 

21 

16 

86 
48 

18 

7-7 

The  torque  is  assumed  to  be  constant  at  all  speeds. 

In  practice  it  will  be  found  that,  in  order  to  give  the 
best  all-round  results,  the  motor  for  potential  control  will 
have  a  lower  efficiency  at  full  speed  than  the  motor  built 
for  rheostatic  speed  control. 

The  motor  with  rheostatic  control  shows  the  same  power 
factor  at  all  speeds. 

The  potential  control  gives  a  lower  power  factor  and 
efficiency  at  all  but  full  speed. 

The  motor  controlled  by  change  of  poles  will  be  found 
to  be  the  most  efficient  for  half  and  quarter  speeds,  and 
has  the  highest  power  factor  except  at  quarter  speeds. 

Of  the  commercial  methods  of  obtaining  speed  variation, 
that  by  potential  control  is  inferior  to  the  rheostatic  con- 
trol in  point  of  efficiency.  The  drawback  to  the  rheostatic 
method  is  that  the  motor  requires  collector  rings. 

Frequency Induction  motors  of  frequencies  of  from 

25  to  60  cycles,  as  constructed  at  the  present  time, 
have  somewhat  better  power  factors  and  efficiencies  than 
higher  frequency  motors.  Motors  of  a  frequency  of  125 
cycles  or  thereabouts  are  seldom  built  in  sizes  above 
20  H.P.  Motors  of  this  frequency  being  somewhat 
difficult  of  construction  on  account  of  the  small  air  gap 


84    POLYPHASE  APPARATUS  AND  SYSTEMS. 

required  and  the  greater  number  of  poles,  are  not  cheaper 
than  25  cycle  or  60  cycle  motors  of  corresponding  sizes, 
as  might  be  expected.  The  reverse  holds  good  with  lower 
frequencies,  60  cycle  motors  costing  less  to  build  than 
motors  of  25  cycles. 

Twenty-five  cycle  motors  have  the  disadvantage  that  on 
account  of  difficulties  in  the  winding  construction,  the 
speeds  are  practically  limited  to  750,  500,  375,  and  300 
R.P.M.  The  bipolar  motor,  running  at  1,500  revolutions, 
is  limited  to  the  smallest  sizes.  The  slow  speeds  of  300 
and  375  revolutions  make  the  motor,  unless  it  be  one  of 
great  capacity,  an  expensive  piece  of  apparatus.  These 
conditions  limit  the  average  practical  speed  of  25  cycle 
motors,  of  sizes  from  5  H.P.  to  75  H.P.,  to  750  revolutions. 

Frequencies  of  35  to  40  cycles  are  more  desirable  for 
the  average  conditions  of  motor  work,  as  they  permit  a 
much  greater  range  of  commercial  speeds. 

The  frequency  of  60  cycles  likewise  permits  the  con- 
struction of  motors  with  a  wide  range  of  speed,  and  which 
are  comparatively  cheap  to  build  throughout  the  entire 
list. 

Voltage Induction  motors  should  not  be  run  at  lower 

voltages  than  that  for  which  they  are  designed,  as  the  out- 
put varies  with  the  square  of  the  voltage.  For  instance, 
if  the  volts  at  the  motor  are  10  per  cent  lower  than  nor- 
mal, a  motor  which  has  a  maximum  output  of  30  per  cent 

greater  than  the  full-load  output  will  give  only  -        -   x 

130=  105  per  cent  of  its  rated  output.  The  margin  is 
too  close  for  continuous  work,  as  it  will  not  take  care  of 
any  sudden  fluctuation  of  load  or  unusual  drop  in  the  line. 
The  output  of  the  motor,  on  a  higher  voltage  circuit  than 


INDUCTION    MOTORS.  85 

that  for  which  it  is  designed,  will  be  increased,  and  the 
current  likewise,  especially  at  light  loads.  Within  ordi- 
nary variation  of  voltages,  the  power  factor  and  efficiency 
at  full  load  remain  practically  unchanged. 

In  laying  out  the  wiring  of  a  motor  which  takes  a  heavy 
starting  current,  allowance  should  be  made  for  this  mo- 
mentary current ;  otherwise  the  impressed  volts  may  drop 
below  the  point  where  the  motor  will  start. 

Motors  with  stationary  fields  could  be  wound  for  fairly 
high  voltage,  but  for  the  distributed  form  of  winding  re- 
quired to  keep  down  self-induction,  the  space  necessary 
for  high  insulation  being  occupied  by  the  conductor. 
Standard  American  motors  below  50  H.P.  are  not  wound 
above  550  volts.  It  is  considered  practical  to  wind  larger 
motors  up  to  3,000  volts.  European  makers,  on  the  other 
hand,  build  motors  of  10  H.P.  to  30  H.P.  for  pressures  of 
500  to  2,000  volts,  motors  of  50  H.P.  for  3,000  volts,  and 
those  of  75  H.P.  and  larger  for  5,000  volts. 

Power  Factor  —  Efficiency It  has  been  seen  that  the 

ratio  of  the  energy  current  of  a  motor,  or  the  current 
required  in  supplying  its  losses  and  doing  the  work  to  the 
total  current  consumed,  gives  the  power  factor.  The  pro- 
duct of  the  power  factor  and  the  actual  efficiency  of  an  in- 
duction motor  gives  the  apparent  efficiency.  This  last 
quantity  determines  the  capacity  of  transformers  and  gen- 
erators required  for  supplying  current  to  the  motors.  As 
has  been  seen,  the  influence  of  the  power  factor  extends 
back  in  the  chain  of  transmission  with  greater  effect  on 
the  supplying  circuit,  necessitating,  in  the  case  of  a  poor 
power  factor,  on  account  of  its  inductive  effects,  an  addi- 
tional increase  in  the  capacity  of  the  transmission  lines. 
For  this  reason,  it  is  usually  of  importance  that  induction 


86 


POLYPHASE   APPARATUS   AND    SYSTEMS. 


motors  be  designed  to  give  the  highest  possible  power  fac- 
tor. Where  the  generating  power  is  expensive,  it  is  some- 
times of  more  importance  to  use  motors  of  higher  efficiency 
than  those  of  high  power  factor.  Under  all  circumstances, 
however,  it  is  desirable  to  have  the  apparent  efficiency  of 
the  motors  as  high  as  possible. 

The  power  factors  of  standard  commercial  induction 
motors  of  American  manufacture  vary  at  full  load  from 
.75  to  .92,  depending  upon  the  size  and  frequency  of  the 
motor.  The  efficiencies  range  from  .80  to  .92.  The 
apparent  efficiencies  in  motors  above  5  H.P.  output  will 
be  found,  as  a  rule,  not  less  than  .75.  This  means  that 
the  transformer,  supplying  current  to  induction  motors  of 
average  sizes,  must  have  a  capacity  of  I  K.W.  for  every 
horse-power  output  of  the  motors. 

The  following  table  gives  approximate  capacities  of 
standard  transformers  that  should  be  used  with  two-phase 
and  three-phase  induction  motors  : 


H.  P. 
CAPACITY  MOTOR. 

THREE-PHASE. 

TWO-PHASE. 
2  TRANSFORMERS. 

2  TRANSFORMERS. 

3  TRANSFORMERS. 

I 

.6  K.W. 

.5  K.W. 

.6  K.W. 

sy 

i.       " 

.1      " 

i.       " 

3 

2             " 

1.5      " 

1.5      " 

5 

3-       " 

0                        " 

->        i' 
3- 

1% 

4-        " 

2-5 

4-        " 

10 

5- 

3-5 

5-        " 

15 

7-5 

5- 

7-5      " 

20 

10.        " 

7-5 

10. 

3° 

I5- 

10.             " 

!5- 

50 

25. 

15.        " 

•7  -                       " 

75 

25.        " 

35-        " 

IOO 

30.        " 

45-        " 

INDUCTION    MOTORS.  87 

The  efficiency  of  commercial  induction  motors  can  be 
somewhat  increased  by  not  sparing  iron  and  copper,  as 
the  losses  of  an  induction  motor  are  of  the  same  kind 
as  those  of  a  generator,  consisting  of  copper  loss,  hyste- 
resis loss,  and  friction  loss. 

The  power  factor  can  be  bettered  by  reducing  the  air 
gap  and  iron  density,  and  thereby  lowering  the  magnetiz- 
ing or  "wattless"  current.  To  do  this,  however,  and 
retain  high  efficiency,  increases  the  cost  of  the  motor,  and 
it  then  becomes  a  question  whether  the  increased  advan- 
tages are  worth  the  extra  expense.  Mechanical  considera- 
tions limit  the  clearance  between  field  and  armature.  Fio:. 

o 

5  5  shows  the  curves  of  efficiency,  power  factor,  and  appa- 
rent efficiency,  as  well  as  torque  and  ampere  input  of  a  175 
H.P.  motor.  At  full  load  the  efficiency  is  91  per  cent,  the 
power  factor  .88,  and  the  apparent  efficiency  80  per  cent. 
The  efficiency  at  half  load  is  as  good  as  that  at  full  load  ; 
and  at  one-quarter  load,  the  efficiency  is  still  well  up,  being 
85  per  cent.  The  break-down  point  is  at  over  twice  full 
load.  The  power  factor  is  highest  at  about  260  H.P., 
being  over  91  per  cent. 

In  many  cases  it  is  desirable  to  design  motors  so  that 
their  maximum  efficiency  occurs  at  about  three-quarters 
load.  This  is  especially  desirable  for  shop  work,  where 
the  driving  motors  are  called  upon  intermittently  to  give 
full  load,  the  average  demand  being  1 5  per  cent  to  30  per 
cent  less  than  the  load  for  which  they  are  rated. 

The  efficiency  of  a  10  H.P.,  60  cycle  motor  with  short- 
circuited  armature  is  shown  in  Fig.  61,  and  also,  for  com- 
parison, the  curves  of  a  variable  resistance  type  having 
high  inductance.  The  efficiency  of  the  variable  resistance 
motor  is  the  higher  at  all  loads  under  full  load,  after 


88 


POLYPHASE   APPARATUS    AND    SYSTEMS. 


which  the  other  motor  is  ahead.     The  break-down  point 
of  the  latter  motor  is  over  200  per  cent  of  full  load. 

Condensers. — Condensers  are  used  to  improve  the 
power  factor  of  circuits  supplying-  current  to  motors  by 
making  the  motors  take  current  in  proportion  to  the  loads. 
The  motors  themselves  are  not  improved,  but  the  wattless 
current  is  offset  by  the  leading  current  supplied  by  the 


Compensator  Type  Motor 
Hiah  Inductance  - 


20      40      CO      80      100     120     140     KJO     180     200    220 

Per  Cent.  Load 
Fig".  61. 

condensers,  and  its  pernicious  influence  confined  to  the 
local  circuit  between  the  condenser  and  the  motor.  Fig. 
62  shows  the  apparent  efficiency  of  a  Stanley  two-phase 
motor  with  and  without  a  condenser,  and  Fig.  63  the  con- 
nection of  motor  and  condensers. 

The  condenser  consists  of  numerous  thin  sheet  conduc- 
tors, separated  by  still  thinner  dialectrics,  the  whole  elec- 
trically connected  to  form  two  conductors.  As  the  size 
of  the  condenser  increases  rapidly  with  a  low  frequency 


INDUCTION    MOTORS. 


89 


and  voltage,  it  is  best  adapted  for  circuits  of  over  100 
cycles,  and  when  motors  are  used  for  not  less  than  500 
volt  circuits. 


EFFICIENCY  AND  POWER  FACTOR 

8         §        £         S         8         3        8 


AMPERES 


SUP 


Single-Phase  Motors Single-phase    induction    motors 

have    only  recently  been   commercially   introduced    on    a 
large  scale.     They  have  the  characteristic  form  of  poly- 


90         POLYPHASE  APPARATUS  AND   SYSTEMS. 


O 


Condenser 


hi 
Starting 

11 

pj,£p 

fj 

r 

1 

^ 

§*>^ 

Fig-.  63. 


INDUCTION    MOTORS. 


phase  motors.  As  the  flow  of  energy  in  the  single-phase 
system  is  not  continuous,  as  in  a  polyphase  system,  their 
capacity  is  less  than  a  polyphase  motor  of  same  dimensions. 
In  respect  to  torque,  power  factor  and  efficiency,  even  the 
best  commercial  motors  are  not  so  good  as  polyphase 
motors.  An  external  starting  arrangement,  sometimes 
called  a  "phase-splitter,"  is  sometimes  used  with  these 
motors,  for  artificially  producing  a  torque  sufficient  to 
enable  them  to  start  from  rest  under  a  partial  load. 


XAAAAVvM/XAAAAA/VAAA/Nl 
Fig.  65. 


The  winding  of  a  two-pole,  single-phase  motor  is  shown 
in  Fig.  64.  It  has  a  two-phase,  interlinked  winding,  the 
common  terminals  being  at  III.  If  two  currents,  having  a 
difference  in  phase,  are  introduced,  the  dead  point  common 
to  all  single-phase  motors  will  be  overcome,  and  the  ar- 
mature will  revolve.  The  displaced  phase  is  produced  by 
a  combined  resistance  and  impedance  coil,  the  outline  con- 
nections of  which  are  shown  in  Fig.  65.  a  and  b  are 
the  main  leads  ;  c  is  a  lead  to  the  common  terminal  of  the 
motor  two-phase  winding.  R  is  a  resistance  and  L  a  chok- 
ing coil.  The  current  passing  through  R  will  differ  in 


92          POLYPHASE   APPARATUS.  AND    SYSTEMS. 

phase  from  that  flowing  through  L,  and  the  motor  will 
start,  when  the  switch  is  thrown,  with  a  torque  dependent 
upon  the  phase  difference.  The  maximum  torque  will  be 
developed  when  the  currents  are  90°  apart.  This,  of 
course,  cannot  be  obtained  with  this  device.  By  replacing 
the  resistance  by  a  condenser,  a  phase  difference  of  90°  or 
over  can  be  obtained,  with  a  correspondingly  increased 
torque,  and  a  decreased  starting  current.  When  the  motor 
reaches  speed,  the  starting  coils  are  cut  out,  and  it  then 
runs  as  a  single-phase  motor. 

The  usual  form  of  motor  is  provided  with  a  starting 
device  that  gives  half -load  torque  at  about  150  per  cent  of 
full-load  current.  Full  load  torque  may  be  obtained  at 
some\vhat  over  twice  full-load  current,  by  a  special  start- 
ing device.  The  advantage  of  the  single-phase  induction 
motor  over  the  single-phase  synchronous  motor  lies  prin- 
cipally in  the  fact  that  the  latter  motor  is  liable  to  be 
thrown  out  of  step  by  any  fluctuation  in  the  generator 
speed.  The  synchronous  motor  is  fairly  efficient,  and  has 
a  power  factor  of  nearly  unity,  but  the  current  at  starting 
is  quite  out  of  proportion  to  the  torque. 

A  three-phase  induction  motor  will  give  about  40  per 
cent  of  its  output  when  used  single-phase.  A  two-phase 
motor  will  give  50  per  cent  of  its  two-phase  rating  under 
the  same  conditions.  The  same  motors  can  be  rewound  as 
single-phase  motors,  and  will  then  have  an  output  of  over 
75  per  cent  of  their  former  rating.  The  unaltered  two- 
phase  and  three-phase  motors  can,  however,  be  made  to 
yield,  on  a  single-phase  circuit,  about  75  per  cent  of  their 
rating  by  increasing  the  voltage  30  per  cent  above  that  for 
which  they  are  wound. 

In    Fig.  66   is    shown    the    connections    of   a   Wagner 


INDUCTION    MOTORS. 


93 


Electric  Company's  self-starting,  single-phase  motor.  In 
starting,  the  armature  and  field  are  connected  in  series. 
On  attaining  full  speed,  this  connection  is  automatically 
broken  by  a  governing  device  within  the  armature.  Simul- 
taneously, the  armature  is  short-circuited  on  itself,  and  the 


Line 


Double  Pole 
Fuse  Block 


Double  Pole 
Knife  Switch 


Fig.  66. 

field  remains  connected  across  the  line.     The  motor  then 
operates  as  a  simple  induction  motor. 

As  seen  in  the  diagram,  no  external  starting  device  is 
required,  there  being  only  two  wires  from  the  mains  to 
the  motor.  The  third  binding  post,  C,  is  for  use  in  case 
the  voltage  of  the  supplying  circuit  is  low,  being  connected 
to  the  supplying  circuit  at  starting  only. 


94          POLYPHASE   APPARATUS   AND    SYSTEMS. 


CHAPTER   V. 
SYNCHRONOUS   MOTORS. 

General.  —  Any  alternating-current  generator,  with  little 
or  no  change,  can  be  used  as  a  synchronous  motor.  Elec- 
trically and  mechanically  the  motor  resembles  the  corre- 
sponding generator,  and  must  be  provided  with  the  same 
station  equipment,  including  some  source  of  exciting  cur- 
rent. The  synchronous  motor,  especially  in  units  of  large 
output,  possesses  a  number  of  features  which  makes  its 
use  at  times  preferable  to  that  of  the  induction  motor. 
Besides  the  advantage  of  an  unvarying  speed  at  all  loads, 
the  power  factor  can  be  altered  at  will  by  changing  the 
exciting  current  and  made  equal  to  unity  at  any  load. 
The  current  can  even  be  made  leading,  to  offset  a  lagging 
current  in  other  parts  of  the  system.  The  synchronous 
motor,  especially  at  low  speeds,  is  cheaper  to  build  than 
the  induction  motor,  and  its  efficiency,  as  a  rule,  will  be 
found  to  be  higher. 

As  a  partial  offset  to  these  advantages,  the  synchronous 
motor  is  not  adapted  for  use  where  a  large  starting  torque 
or  frequent  starting  of  the  load  is  necessary.  It  does  not 
admit  of  independent  speed  regulation.  It  also  has  the 
disadvantage  of  requiring  certain  station  appliances  and 
an  exciting  current.  •  Another  objection  to  the  synchro- 
nous motor  is  its  tendency  to  Jjtmt.  It  requires  skillful 
and  intelligent  attention. 


SYNCHRONOUS    MOTORS.  95 

Speed.  —  The  speed  of  the  synchronous  motor  is  not 
necessarily  the  generator  speed,  but  a  speed  which,  mul- 
tiplied by  the  number  of  poles,  gives  a  product  equal  to 
the  generator  alternations.  A  motor,  with  twice  as  many 
poles  as  the  generator,  will  have  half  its  speed,  or  vice 
versa.  As  load  is  thrown  on  the  synchronous  motor, 
there  is  a  lag  in  the  relative  positions  of  armature  winding 
and  pole  face,  or  retardation  of  the  armature.  The  effective 
counter  E.M.F '.  is  thereby  reduced,  which  gives  rise  to  a 
larger  flow  of  current. 

The  motor  speed  is  independent  of  the  voltage  and  can- 
not be  altered  except  by  changing  the  generator  speed. 
It  is  important  therefore  that  the  regulation  of  the  prime 
mover  be  as  perfect  as  possible,  both  in  the  number  of  rev- 
olutions per  minute  and  in  the  angular  speed;  otherwise, 
as  the  fly-wheel  capacity  of  the  motor  armature  is  sufficient 
to  absorb  considerable  energy  without  changing  its  speed, 
fluctuating  currents  will  pass  between  generator  and  motor, 
reducing  the  motor  capacity,  and  producing  bad  regulation. 

Torque  and  Output.  —  A  synchronous  motor  at  starting 
acts  somewhat  as  an  induction  motor.  Consequently  any 
variation  of  its  proportions,  such  as  the  shape  of  the  pola 
pieces,  armature  reaction,  and  nature  of  the  winding, —  i.  e., 
distributed  or  unitooth,  —  affects  its  starting  torque.  The 
starting  torque  may  vary  from  nothing  to  20  or  30  per 
cent  of  full-load  running  torque,  depending  upon  the  motor 
design.  When  once  in  motion,  the  motor  will  rapidly 
attain  synchronous  speed.  Polyphase  motors,  as  usually 
constructed,  will  carry  four  to  five  times  full  load.  If 
further  loaded,  they  fall  out  of  synchronism,  and  can  be 
brought  up  to  speed  by  being  relieved  of  the  load.  Single- 
phase  synchronous  motors  have  dead  points,  and  will  not 
start  from  rest;  monocyclic  generators  used  *  as  motors 


96    POLYPHASE  APPARATUS  AND  SYSTEMS. 

develop  too  feeble  a  torque  to  start,  and  may  be  regarded 
as  single-phase  motors  in  their  action  at  starting,  and  when 
running  under  load.  It  is  necessary  to  use  some  extra- 
neous source  of  power  to  start  single-phase  motors,  and 
bring  them  up  to  speed.  This  is  usually  effected  by  an 
alternating-current  motor.  In  some  cases  where  a  direct- 
current  source  of  power  is  available,  the  exciter  may  be 
used  as  a  starting  motor. 

.  The  limit  to  the  torque  and  output  of  a  synchronous 
motor  is  dependent  mainly  upon  the  terminal  voltage. 
Under  rated  voltage  the  margin  of  most  motors,  before 
the  break-down  point  is  reached,  is  sufficient  to  enable 
them  to  stand  a  heavy  overload.  Variation  of  the  speed 
of  the  prime  mover  will  reduce  the  maximum  output. 

Voltage The  relation  of  impressed  volts  to  the  max- 
imum output  is  the  same  in  synchronous  as  in  induction 
motors,  the  output  and  the  starting  torque  varying  within 
certain  limits  as  the  square  of  the  volts.  It  is  essential, 
therefore,  that  the  pressure  be  kept  at  the  rated  voltage  of 
the  motor;  otherwise  the  motor  may  not  start  at  all,  par- 
ticularly if  it  consumes  an  excessive  starting  current. 

Synchronous  motors  can  be  wound  for  the  same  volt- 
age as  the  corresponding  generators.  Standard  motors  of 
too  H.P.  and  over,  of  the  revolving  armature  type,  are 
wound  for  potentials  up  to  3,400  volts.  Motors  of  the 
stationary  armature  type  can  be  safely  wound  for  poten- 
tials as  high  as  7,000  volts,  in  sizes  from  100  to  500  K.W.; 
motors  of  larger  capacity  can  be  wound  for  15,000  volts. 
Motors  of  the  revolving  field  type,  as  ordinarily  propor- 
tioned, have  a  somewhat  greater  starting  torque  than  those 
of  the  revolving  armature  type,  on  account  of  the  greater 
arc  covered  by  the  pole  face. 


SYNCHRONOUS    MOTORS.  97 

Methods  of  Starting When  a  large  torque  is  required 

to  turn  over  the  load,  as  in  the  case  of  mill  machinery  or 
long  lines  of  shafting  and  belting,  a  friction  clutch  must 
be  used.  This  permits  the  load  to  be  gradually  thrown  on 
the  motor  after  it  reaches  synchronism.  The  clutch  may 
be  mounted  on  the  motor  base  extended,  an  extra  standard 
being  required  for  this  purpose ;  or  the  motor  may  be 
belted  to  a  line  shaft  on  which  there  is  a  coupling.  This 
is  the  cheaper  and  more  usual  method.  Fig.  67  shows  a 
500  H.P.  motor,  built  with  extended  base,  carrying  a  clutch 
and  driving  pulley.  In  selecting  a  coupling  for  this  class 
of  work,  one  of  ample  proportions  should  be  used,  as  it 
must  start  the  load  gradually,  without  exceeding  the  break- 
down point  of  the  motor,  and  without  overheating. 

The  operations  in  starting  a  synchronous  motor  are 
about  as  follows  :  First,  the  main  switch  is  closed  and  the 
motor  with  its  fields  unexcited  will  start  with  a  small 
torque  due  to  the  induced  currents  in  the  pole  pieces,  and 
soon  speed  up  to  almost  synchronism.  The  current  from 
the  exciter,  which  is  either  belted  to  or  mounted  on  the 
motor  shaft,  can  now  be  switched  into  the  fields,  and  the 
motor  will  be  brought  up  to  synchronism.  The  full  load 
can  then  safely  be  thrown  on  the  motor  by  the  friction 
clutch,  if  one  is  used. 

The  current  taken  at  starting  may  be  anything  from  150 
per  cent  of  full-load  current  to  several  times  normal  cur- 
rent, being  limited  by  the  resistance  and  self-induction  of 
the  armature  windings,  i.  e.,  its  impedance.  This  exces- 
sive starting  current,  as  it  is  of  an  inductive  character,  may 
cause  a  large  drop  in  the  line,  and  disarrange  the  voltage 
of  the  entire  system. 

If  the  motor  takes  a  large  proportion  of  the  generator 


POLYPHASE  APPARATUS  AND   SYSTEMS. 


SYNCHRONOUS    MOTORS. 


99 


Double-throw  Switch 


output,  or  is  used  in  connection  with  lights,  and  started  and 
stopped  at  frequent  intervals,  some  other  means  should  be 
employed  to  reduce  the  current.  This  can  be  done,  as  in 
the  case  of  the  induction  motor,  by  the  use  of  a  resistance, 
a  reactance,  or  a  compensator  in  the  main  circuit,  or  by  a 
small  starting  motor.  A  compensator  starter,  like  that 
shown  in  Fig.  68, 

Running  s/cfe- 


is  sometimes  used. 
This  particular 
starting  device 
closely  resembles 
the  compensator 
starter  for  three- 
phase  induction 
motors,  shown  in 
Fig.  49.  It  is  pro- 
vided with  three 
taps,  giving  volt- 
ages 40  per  cent, 
50  per  cent,  and 
60  per  cent  of  run- 
ning full-load  volt- 
age. With  50  per  cent  of  the  impressed  volts,  the  syn- 
chronous motor,  when  properly  proportioned,  will  take,  at 
starting,  a  current  equal  to  about  full-load  current,  and 
start  with  a  torque  about  15  per  cent  of  the  full-load  run- 
ning torque.  The  operation  of  this  starting  device  is 
plainly  indicated.  The  triple  pole  switch  is  down  at  the 
moment  of  starting,  and,  when  nearly  synchronous  speed 
is  reached,  is  thrown  up  to  the  running  side. 

A  starting    motor,   of    the   induction  type,   is   the  best 
means  of  reducing  the  current  at   starting.     The  current 


Continuous  Winding 

Fig.  68. 


100   POLYPHASE  APPARATUS  AND  SYSTEMS. 

taken  by  this  small  motor  cannot  seriously  affect  the  volt- 
age of  the  circuit.  This  method  should  be  employed  when 
the  motor  is  started  frequently,  or  when  a  low  starting  cur- 
rent is  essential  to  preserve  good  regulation.  A  starting 
motor,  one-tenth  the  capacity  of  the  synchronous  motor, 
will  usually  be  found  of  sufficient  size  to  meet  all  average 
conditions.  When  an  auxiliary  motor  is  used,  the  synchro- 
nous motor  must  both  be  brought  up  to  slightly  above  syn- 
chronous speed,  and  the  speed  of  the  motor  E.M.F.  brought 
into  opposition  with  the  generator  E.M.F.  Fig.  69  shows 
a  1000  H.P.  two-phase  motor  with  induction  motor  starter. 

Many  forms  of  self-starting  synchronous  motors  have 
been  devised  for  use  on  single-phase  circuits.  Most  of 
these  are  provided  with  a  commutator  for  self-excitation, 
and  a  starting  device.  A  commutator,  in  series  with  the 
field  winding,  rectifies  the  current  at  the  instant  the  main 
armature  current  is  in  phase  for  producing  a  slight  torque. 
When  the  motor  reaches  speed,  the  commutator  is  cut  out. 
One  of  these  types  is  the  single-phase  motor  made  by  the 
Fort  Wayne  Company,  which  embodies  a  modification  of 
this  construction.  The  main  current  is  first  thrown  on  a 
continuous  current  winding  connected  to  a  commutator, 
and  laid  over  the  alternating-current  winding  on  the  arma- 
ture, which  is  connected  to  collector  rings.  When  the  motor 
reaches  synchronism,  the  main  current  is  switched  into  the 
alternating-current  winding,  and  the  field  circuit  closed  on 
the  starting  winding  through  the  commutator. 

In  starting  a  synchronous  motor,  difficulty  is  sometimes 
encountered  in  the  high  voltage  induced  in  the  fields  by 
the  armature  current.  This  is  overcome  in  the  revolving 
field  type  of  motor  by  using  an  exciting  current  of  low 
potential,  —  sometimes  as  low  as  50  volts,  and  in  the  re- 


SYNCHRONOUS    MOTORS. 


101 


102   POLYPHASE  APPARATUS  AND  SYSTEMS. 

volving  armature  type  by  breaking  up  the  fields  into  a 
number  of  parts,  or  by  open-circuiting  each  field  spool,  as 
shown  in  Fig.  70.  Leads  from  each  spool  are  brought  out 
to  convenient  switches  on  the  motor  frame.  The  motor  is 
started  with  these  open.  When  synchronism  is  reached 
the  switches  are  closed,  thus  putting  the  field  coils  in  series, 
and  throwing:  them  in  circuit  with  the  exciter. 


Fig.  70. 

Field  Excitation.  —  An  increase  of  the  field  excitation 
of  the  synchronous  motor  will  cause  a  corresponding  in- 
crease in  the  E.M.F.  generated  in  the  motor.  By  properly 
proportioning  the  field  excitation,  this  E.M.F.  of  the  motor 
can  be  made  considerably  greater  than  the  impressed  volts 
at  the  motor  terminals.  It  will  be  seen  that  an  opposite 
condition  exists  from  that  when  the  induced  E.M.F.  is 
small,  due  to  a  small  exciting  current.  In  the  first  case, 
the  phase  of  the  current  will  be  found  to  be  in  advance  of 
the  impressed  volts,  and  in  the  second  case,  to  be  lagging 
behind.  It  follows,  then,  that  for  any  condition  of  load  of 
the  synchronous  motor,  by  simply  changing  the  strength 


SYNCHRONOUS    MOTORS. 


103 


of  the  exciting  current,  the  armature  current  can  be  made 
lagging,  in  phase  with,  or  in  advance  of,  the  impressed 
E.M.F.  In  other  words,  the  amount  of  current  consumed 
by  the  motor  depends  upon  the  field  excitation. 

The  effect  upon  the  armature  current,  produced  by  vary- 
ing the  field  excitation,  is  shown  by  the  curves  in  Fig.  7 1 . 
Up  to  a  certain  point,  as  the  excitation  is  increased,  the  ar- 
mature current  is  lagging,  and  decreases.  Further  increase 
of  the  exciting  current  causes  the  armature  to  consume  more 


Exciting  Current 
Fig-.  71. 

current,  which  is  now  leading.  There  is  one  value  of  the 
exciting  current  for  which  the  armature  current  is  a  mini- 
mum. In  motors  of  good  regulation  this  value  varies  but 
slightly  with  different  loads. 

The  result  obtained  from  this  property  of  the  synchron- 
ous motor,  of  producing  at  will  any  displacement  of  phase 
between  current  and  E.M.F.,  is  the  possibility  of  annulling 
the  reactance  due  to  the  inductance  of  the  line,  and  at  the 
same  time  of  compensating  for  a  certain  amount  of /lagging 
current  due  to  inductive  loads  in  other  parts  of.tJ^e  circuit- 


104   POLYPHASE  APPARATUS  AND  SYSTEMS. 


When  over-excited,  the  synchronous  motor  acts  like  a 
great  condenser.  It  will  take  care  of  a  total  current 
made  up  of  energy  and  wattless  components,  to  an  ex- 
tent equal  to  its  rated  ampere  output. 

Synchronous  motors  of  the  polyphase  type  are  separately 
excited.  No  series  winding  or  automatic  compounding  is 
required. 


Exciting  Current  and  Exciters  for  Standard  Polyphase  Genera- 
tors and  Synchronous  Motors. 


25-GO  ( 

CYCLES. 

GENERATOR  OR  MOTOR. 
RATING  K.  W. 

SEPARATE 
EXCITING 
CURRENT. 

VOLTAGE. 

EXCITER 
CAPACITY. 

(  Generator  .  . 
I  Motor    .... 

-     6 

10 

125 

I25 

iy2  K.W. 
2     K.W.  to  2y2  K.W. 

j  Generator  .  . 
1    1  Motor    .... 

8 
13 

25 
25 

2     K.W.  to  2y2  K.W. 

2i/2  K.W. 

IOO-)Generator  •  ' 
{  Motor    .... 

10 

15 

25 
25 

214  K.W. 
2y2  K.W. 

T        f  Generator  .  . 
1  Motor    .... 

12 
2O 

25 
25 

2y2  K.W. 
y/2  K.W.  to  4%  K.W. 

j  Generator  .  . 
"5      I  Motor    .... 

19 

25 

4y2  K.W. 
7y2  K.W. 

When  generators  provided  with  automatic  compounding 
are  converted  into  synchronous  motors,  it  will  be  found 
that  increased  separate  excitation  is  required  ;  since  the 
series  fields  are  necessarily  omitted  in  the  motor.  The 
standard  exciters  usually  furnished  with  generators  under 
250  K.W.  capacity  must,  as  a  rule,  be  replaced  by  the 
next  larger  sizes.  The  preceding  table  gives  the  average 
separate  exciting  current  required  by  standard  polyphase 
generators  and  motors  of  moderate  output,  power  factor 


SYNCHRONOUS    MOTORS.  105 

being  taken  as  unity.  The  exciter  capacities  given  are 
sufficient  to  take  care  of  inductive  conditions.  The  ex- 
citers should  have  more  capacity  than  is  actually  required, 
as  they  are  the  weakest  part  of  the  system,  and  should  not 
be  taxed  to  their  full  capacity.  In  a  station  where  several 
synchronous  motors  or  generators  are  used,  it  is  customary 
to  install  two  exciting  dynamos,  each  one  of  which  has 
capacity  to  furnish  sufficient  excitation  for  all  machines. 

Power  Factor.  —  The  maximum  efficiency  of  the  motor 
and  circuit  exists  when  the  current  and  E.M.F.  supplied 
to  the  motor  are  in  phase,  —  i.e.,  when  the  power  factor  is 
unity.  This  is  also  a  condition  of  minimum  current,  and 
the  drop  in  the  line  is  that  due  to  ohmic  resistance  only. 
When  the  current  is  in  advance  of,  or  lagging  behind  the 
impressed  volts,  the  power  factor  is  less  than  unity.  It  is 
possible,  as  we  have  seen,  to  suitably  proportion  the  excit- 
ing current  of  a  synchronous  motor,  so  that  its  power 
factor  may  be  unity  at  any  load.  In  this  way  a  low  power 
factor  of  the  supplying  circuit,  due  to  induction  motors, 
may  be  raised  any  amount. 

On  account  of  armature  reaction,  a  motor,  which  has  its 
excitation  adjusted  to  give  a  power  factor  of  unity  at  full 
load,  will  take  a  leading  current,  and  have  a  power  factor 
less  than  100  per  cent  at  all  points  below  full  load.  For 
the  average  case,  it  will  be  found  most  desirable  to  so 
excite  the  motor  fields  that  the  minimum  current  and 
highest  power  factor  are  reached  at  about  average  load. 
The  power  factor  will  be  leading  at  lighter,  and  lagging  at 
greater,  loads.  Except  in  the  case  of  synchronous  motors 
of  abnormally  bad  design,  the  power  factor,  with  properly 
excited  fields,  will  have  a  high  value  over  a  wide  range  of 
load.  Even  motors  with  considerable  armature  reaction 
will  have  only  a  slightly  drooping  curve  of  power  factor. 


106   POLYPHASE  APPARATUS  AND  SYSTEMS. 

Motors  which,  as  generators,  would  have  excellent  in- 
herent regulation,  —  i.e.,  small  armature  reaction  and  self- 
induction, —  can  be  made  to  have,  with  one  adjustment  of 
the  field,  practically  100  per  cent  power  factor  at  all  but 
light  loads.  The  advantage  of  a  more  uniform  power 
factor  in  such  motors  is  offset  by  their  instability  during 
voltage  fluctuations.  Some  self-induction  is  desirable  in 
order  to  prevent  exchange  of  current  between  motor  and 
lines  when  the  impressed  volts  vary,  as  often  happens  in 
power  transmissions.  In  selecting  a  synchronous  motor, 
therefore,  preference  should  be  given  to  that  one  which,  as 
a  generator,  would  not  have  very  close  inherent  regulation. 
Machines,  of  not  such  good  regulation,  have,  as  a  rule,  a 
higher  efficiency,  and  take  less  starting  current. 

To  predetermine  the  proper  field  strength  which  will 
give  the  maximum  condition  of  efficiency,  it  is  necessary 
to  know  the  conditions  of  the  system,  —  the  reactance  of 
the  generator  and  line,  the  average  load  and  its  power  fac- 
tor, and  the  characteristics  of  the  motor.  Each  case  is  a 
problem  by  itself,  and  must  be  judged  by  the  special  con- 
ditions affecting  it. 

A  synchronous  motor  will  take  no  more  than  its  rated 
amperes  without  overheating,  whatever  the  phase  relation 
of  current  and  E.M.F,  may  be.  If  the  inductive  load  at 
the  receiving  end  is  large,  as  compared  with  the  motor 
load,  the  synchronous  motor  may  prove  inadequate  to  carry 
its  own  load,  and  appreciably  annul  the  inductive  effects. 

It  will  be  found  that,  for  every  load  and  every  power 
factor,  there  is  a  synchronous  motor  capacity  which  will 
make  the  efficiency  of  the  system  ?.  maximum.  Mr.  E.  J. 
Berg  has  calculated  the  influence  of  synchronous  motors 
upon  the  efficiency  of  alternating  systems.  Fig.  72  shows 


SYNCHRONOUS    MOTORS. 


JO/ 


the  different  efficiencies  of  a  transmission  of  a  constant 
current  of  200  amperes  when  a  synchronous  motor  of  50, 
100,  or  150  K.W.  is  running  as  a  compensator  at  the 
receiving  end,  which  is  assumed  to  have  varying  power 


i)0  SO  70  GO          50          40  30          20          10  0 

POWER  FACTOR  (RECEIVING  END)  ' 
Fig-.  72. 

factors.     The  circuit   is   supposed  to  have    the    following 
constants  :  — 

Current  =  200  amperes. 

Resistance  =  .52  ohms. 

Reactance  .=  1.45  ohms. 

Voltage  at  motor  =  1000. 

It  will  be  noticed  that,  as  the  power  factor  diminishes  at 
the  receiving  end,  the  line  efficiency  is  increased  by  using 
the  larger  synchronous  motors  —  i.e.,  will  transmit  a  great 


108   POLYPHASE  APPARATUS  AND  SYSTEMS. 

amount  of  energy  for  the  same  loss.  The  line  efficiency  is 
greatest  when  using  the  150  K.W.  motor  at  all  power 
factors  below  87.  The  line  efficiency  is  improved  by  en- 
tirely dispensing  with  the  motors  when  the  power  factor  is 
greater  than  95.  The  leading  current  of  the  motors  is 
then  in  excess  of  the  lagging  current  of  the  receiving  cir- 
cuit, thereby  increasing  the  total  current,  or  when  main- 
taining a  constant  current,  as  in  the  present  case,  decreasing 
the  energy  current  —  i.e.,  the  amount  of  power  that  can  be 
transmitted  over  the  lines  with  the  conditions  as  given. 

Instability.  —  When  a  number  of  synchronous  motors* 
are  running  at  the  end  of  a  long  transmission  line  it  is 
sometimes  found  that  a  condition  of  instability  exists,  espe- 
cially during  load  variations.  This  is  shown  by  an  inter- 
change of  synchronizing  current.  This  difficulty  may  in 
part  be  overcome  by  under  exciting  the  motor  fields,  which 
gives  a  lagging  armature  current. 

The  instability  or  "  hunting  "  of  synchronous  motors 
may  also  be  caused  by  the  periodic  variation  in  the  angu- 
lar velocity  of  the  prime  mover.  The  permissible  limits 
of  angular  variation  have  been  fixed  principally  because 
of  the  effect  on  the  operation  of  synchronous  apparatus  of 
changes  in  frequency  of  the  supply  current  during  each 
revolution  of  the  generator.  Any  disturbance  distorting 
the  motor-field  flux,  thereby  pulling  ahead  or  retarding  the 
revolving  element,  will  increase  the  tendency  to  hunt. 
This  tendency  is  more  noticeable  in  motors  of  high  volt- 
age when  designed  with  armatures  having  few  slots.  It 
may  be  reduced  by  various  forms  of  anti-hunting  devices, 
such  as  short-circuited  bars  piercing  the  pole  pieces,  or  by 
copper  rings  surrounding  the  poles,  or,  better  still,  by  cop- 
per or  heavy  aluminum  bridges  between  adjacent  poles. 


ROTARY   CONVERTERS.  109 


CHAPTER   VI. 
ROTARY   CONVERTERS. 

General.  —  Any  direct-current  generator  can  be  used  as 
a  rotary  converter  by  tapping  the  armature  windings  at 
particular  points  and  connecting  the  leads  to  collector 
rings.  Direct  current  can  be  taken  from  the  brushes  of 
the  machine  at  the  commutator  end,  if  an  alternating  cur- 
rent is  supplied  to  the  collector,  or  vice  versa.  If  connec- 
tions are  made  with  the  armature  at  points  differing  from 
each  other  by  180  electrical  degrees,  the  machine  becomes 
a  single-phase  rotary  converter  ;  while  connections  at  points 
90°  apart  will  give  a  two-phase  relationship.  Connections 
made  at  points  120  electrical  degrees  apart  permit  the  use 
of  the  machine  on  three-phase  circuits. 

The  output  of  such  a  machine  is  increased  when  used  as 
a  rotary  converter.  This  is  partly  due  to  the  absence  of 
armature  reaction.  The  direct  current  flowing  out  may 
be  said  to  neutralize  the  armature  reaction  of  the  alternat- 
ing current  flowing  in.  Again,  at  certain  positions  of  the 
armature,  the  current  flows  through  the  shortest  possible 
path  from  collector  to  commutator.  When  used  as  a  motor, 
taking  current  from  either  the  direct  or  alternating  current 
end,  a  rotary  will  heat  more  for  the  same  current  input  than 
when  used  solely  for  the  conversion  of  the  current. 

While  a  direct-current  generator  may  be  made  into  a 
rotary  converter  in  the  manner  described,  it  is  not  desir 


110       POLYPHASE    APPARATUS    AND    SYSTEMS. 

able  to  do  so,  on  account  of  the  low  frequency  which  such 
a  machine  will  have.  It  does  not  follow,  moreover,  that 
the  direct-current  generator  will  fulfil  the  conditions  of 
successful  commercial  operation.  On  the  contrary,  it  is 
probable  that,  without  .some  .  change  in  the  proportioning 
of  parts  and  windings,  such  a  rotary  converter  would  be  a 
failure. 

As  usually  designed,  rotary  converters  vary  but  little  in 
mechanical  construction  and  in  general  appearance  from 
direct-current  generators.  Fig.  73  illustrates  a  400  .K.W. 
Westinghouse  converter.  This  machine  as  shown  is  pro- 
vided with  an  induction-starting  motor,  which  is  used  when 
the  converter  is  started  from  the  alternating-current  end, 
and,  like  the  similar  motor  in  a  synchronous  motor,  reduces 
the  starting  current.  No  pulley  is  provided,  unless  it  is 
intended  to  operate  the  rotiry  as  a  double-end  generator  or 
as  a  motor.  The  armature  is  usually  of  large  diameter,  to 
give  efficient  ventilation.  That  of  a  600  K.W.  rotary, 
recently  built,  has  the  high  peripheral  speed  of  7,500  feet 
per  minute. 

Connections.  —  A  number  of  connections  of  the  alter- 
nating end  of  rotary  converters. are  diagramatically  shown 
in  Figs.  74  to  78.  Fig.  74  is  a  single-phase  arrangement, 
The  armature  windings  are  tapped  at  two  opposite  points, 
and  leads  are  brought  out  to  two  collector  rings.  The 
connections  for  three-phase  rotary  converters  are  shown  in 
Fig.  75.  The  three  collector  rings  are  connected  to  three 
points  in  the  armature,  120°  apart.  Fig.  76  illustrates  the 
usual  method  of  making  connections  for  a  two-phase  rotary 
converter.  These  are  the  simple  connections  of  bipolar 
machines.  In  multipolar  rotary  converters,  the  collector 
rings  are  connected  to  as  many  points  of  the  armature  as 


ROTARY    CONVKRTERS, 


I  1 1 


112       POLYPHASE    APPARATUS    AND    SYSTEMS. 

there  are  pairs  of  poles,  —  i.e.,  the  connections  must  be 
duplicated  for  each^  360  electrical  degrees  of  the  machine. 
Fig.  77  shows  the  connections  of  a  four-pole  single-phase 
rotary.  The  two  pairs  of  leads  run  from  points  of  the 
armature  winding,  180  electrical  degrees  apart. 


Fig.  74. 


Fig.  76. 


Fig-.  77. 


It  has  been  noticed  that  the  increased  output  of  a 
machine,  when  used  as  a  rotary  converter,  is  partly  due  to 
some  of  the  current  passing  directly  from  collectors  to 
commutator.  The  output  can  be  made  still  greater  by 
increasing  the  number  of  collector  rings  and  connections. 
For  instance,  a  three-phase  arrangement  can  be  made  with 


ROTARY   CONVERTERS. 


six  collector  rings,  as  in  Fig.  75,  and  a  two-phase,  with 
eight  collector  rings.  In  the  three-phase  arrangement, 
the  phases  are  not  interlinked  at  the  collector  rings.  , 

Ratio  of  Alternating  to  Direct-Current  Voltage. — The 
voltage  of  the  alternating  current  of  a  rotary  converter  is 
always  less  than  that  of  the  direct-current  end,  the  value 
of  which  is  equal  to  the  crest  of  the  E.M.F.  wave,  while 
the  alternating  pressure  is  rated  by  the  mean  effective 
value.  The  natural  E.M.F. 
of  a  direct-current  generator 
is  alternating  in  character, 
and  is  rectified  by  the  com- 
mutator when  the  impulses 
are  at  their  maximum.  The 
measured  or  effective  value 
of  this  unrectified  E.M.F.  is 

— -  =  .707  of  the  E.M.F.,  at 

V2  Fig-.  78. 

the     commutator      brushes. 

This  is  the  relation  between  the  alternating  and  direct 
volts  of  a  single-phase  and  of  a  two-phase  rotary  con- 
verter. From  the  nature  of  the  three-phase  system  cf 
electro-motive  forces,  the  ratio  of  voltages  in  a  three-phase 

rotary  converter  is       3    —  .61 3  of  the  direct  current  E.M.F. 

2  V2 

The  ratio  of  voltage  for  any  particular  converter  cannot  be 
appreciably  varied. 

The  theoretical  ratios  are  not  always  found  in  practice, 
due  to  a  variety  of  causes,  the  main  ones  being  the  depar- 
ture of  the  generator  voltage  from  a  true  sine-wave,  affect- 
ing the  mean  value  of  the  alternating  E.M.F.,  the  drop  in 
the  machine,  which  may  be  i  or  2  per  cent,  the  position  of 
the  direct  current  brushes  on  the  commutator,  the  excita- 


114      POLYPHASE    APPARATUS    AND    SYSTEMS. 

tion,  the  ratio  of  pole  arc  to  pitch,  and  the  conditions  of 
operation. 

As  the  direct  current  voltage  at  the  commutator  brushes, 
neglecting  the  ohmrc  drop  in  the  converter,  is  equal  to  the 
maximum  instantaneous  voltage  at  the  collector  rings,  a 
flat  top  wave  gives  a  higher  ratio,  i.e.,  lower  direct  current 
voltage,  and  a  peaked  wave  a  lower  ratio,  i.e.,  higher  direct 
current  voltage  with  the  same  impressed  voltage.  Further, 
the  shape  of  the  E.HLF.  wave  impressed  by  the  generator 
upon, the  converter  is  modified  by  the  counter  E.M.R  wave 
of  the  converter.  A  short  pole  arc  of  the  converter,  pro- 
ducing a  flat  top  counter  E.M.F.  wave,  here  tends  to  lower 
the  direct  current,  and  a  long  pole  arc  tends  to  raise  the 
direct  current  voltage  at  the  same  impressed  alternating 
voltage. 

A  bad  or  trailing  position  of  the  brushes  from  the  neu- 
tral point  increases  the  ratio,  the  variation  in  extreme 
cases  amounting  to  several  per  cent. 

Over  excitation  may  reduce  the  ratio  one  or  two  per 
cent,  while  with  under  excitation,  i.e.,  lagging  current,  the 
ratio  may  be  increased  the  same  amount. 

Under  average  conditions  of  full  load  operation  the 
standard  types  of  converters  have  ratios  about  as  be- 
low :  — 

Percentage  pole  arc  : 
Three-phase, 


Two-phase, 


^     Ctl  \^     • 

50% 

67% 

74%    • 

So% 

(  55°'v°lt, 

67 

63 

62 

6l.5 

<    250-Volt, 

67.5 

63-5 

62.5 

62 

(  i25-volt, 

68 

63.8 

63 

62.5 

(  55o-volt, 

73 

73-5- 

72'5 

72 

?   250-VOlt, 

79 

74 

73 

72-5 

(    125-VOlt, 

79-5 

74-5 

73-5 

73 

ROTARY    CONVERTERS.  115 

In  the  normal  operation  of  the  converter,  i.e.,  when 
furnishing  direct  current,  the  ohmic  drop  reduces  the  direct 
current  voltage.  When  run  as  an  inverted  converter,  i.e., 
when  delivering  alternating  current,  —  direct  current  being 
fed  to  the  brushes,  —  the  drop  is  on  the  alternating  side  ; 
consequently  the  ratio  of  a  converter  is  lower  when  run 
inverted. 

For  preliminary  calculations  where  the  data  of  operation 
are  not  known,  the  following  ratios  may  be  used  with  most 
standard  converters,  —  for  two-phase,  74  ;  for  three-phase, 
64.5.  It  is  customary  to  make  allowance  for  the  departure 
from  the  normal  ratio,  which  will  be  found  in  the  actual 
operation  of  rotary  converters,  the  extent  of  which  cannot 
always  be  predetermined,  by  using  transformers  with  t^ps 
in  the  secondary  windings,  permitting  a  voltage  chaise  of 
about  5%. 

Six-phase  Converters The  greater  the  number  of  equi- 
distant points  at  which  the  armature  winding  is  tapped,  the 
more  direct  is  the  path  from  the  alternating  current  col- 
lector rings  to  the  commutator,  and  consequently  the  less 
the  heating  of  the  windings.  For  this  reason  a  two- 
phase  converter  of  given  dimensions  has  a  greater  capacity 
than  a  three-phase  and  a  six-phase  a  still  greater  output. 
The  increased  capacity  of  a  six-phase  converter  over 
a  three-phase  may  be  as  much  as  40  to  50  per  cent. 
In  addition  to  the  cheaper  cost  of  manufacture  for  the 
same  output,  a  six-phase  converter  has  the  advantage  of 
superior  commutation.  Its  synchronizing  power  is  also 
greater. 

The  ratios  of  conversion  of  six-phase  converters  depend- 
ing on  the  method  of  connecting  the  secondaries  of  the 
supplying  transformers  will  be  the  same  as  those  for  either 


Il6   POLYPHASE  APPARATUS  AND  SYSTEMS. 

two-phase  or  three-phase  machines.  The  transformers  are 
provided  with  secondaries  divided  into  two  separate  wind- 
ings, and  permit  the  six-phase  arrangement  from  three- 


.2         .4          .0          .S         1.0         1.2        1.4        1.0        l.«        ^.0        2.2 

AMPERES  FIELD 

Fig-.  79. 

phase  currents,  as  is  explained  in  the  chapter  on    three- 
phase  systems. 

Types  of  Converters  determined  by  Field  Excitation.  - 
Rotary   converters   may  be    either   separately   excited   or 
have  both    series  and   separate  field  excitation,  —  i.e.,  be 
shunt   or  compound  wound.     A  third  type  is  sometimes 
constructed,  which  has  neither  separately  nor  series  excited 


ROTARY    CONVERTERS.  I  I/ 

fields,  but  in  which  the  magnetic  field  is  induced  by  the 
armature  current.  This  type  is  known  as  the  "Induction" 
converter,  and  has  the  characteristic  of  an  induction  mo- 
tor, of  a  lagging  current  at  all  loads.  It  runs,  however,  at 
a  synchronous  speed. 

The  current  for  the  separately  excited  fields  is  usually 
supplied  from  the  direct-current  end,  so  no  exciter  is  re- 
quired. The  shunt-wound  rotary  can  be  made  to  give 


2500 
2000 
1500 
1000 

500 

( 

!•    1 

550 

Volts  L 

.  C   Ci 

nstant  Potential 

)          20         40         60          80        100        120        UO        1GO        IfeU        200        22 

AmperesD.  C< 

Fig-.  80 

any  power  factor,  either  leading  or  lagging,  by  over  or 
under  exciting  the  fields.  The  power  factor  will  remain 
practically  constant  for  all  loads.  This  property  is  graph- 
ically shown  in  Fig.  79.  Each  curve  represents  the 
variation  in  the  current  input  at  the  alternating  end,  for 
varying  field  strengths  at  different  loads,  of  a  100  K.W. 
rotary  converter.  The  field  strength  for  minimum  current, 
or  100  per  cent  power  factor,  is  9.2  amperes  at  no  load, 
and  9.55  amperes  at  full  load  of  182  amperes,  proving  that 
the  armature  reaction  is  very  slight. 


Il8   POLYPHASE  APPARATUS  AND  SYSTEMS. 

Fig.  80  shows,  in  another  way,  the  insignificance  of  the 
armature  reaction.  The  ampere  turns  at  no  load  are  2,700, 
and  at  full  load  2,790,  an  increase  of  about  3  per  cent. 

The  shunt-wound  converter  is  particularly  adapted  for 
large  installations  where  load  fluctuations  are  gradual,  and 
good  regulation  and  constant  power  factor  are  of  importance. 

Compound-wound  converters  are  used  to  advantage,  as 
will  be  shown,  for  supplying  current  to  fluctuating  cir- 
cuits, as  in  railway  service,  and  in  cases  where  it  is  neces- 
sary to  maintain  constant  or  increasing  voltage  with  in- 
creasing load.  Various  combinations  of  field  excitations 
are  possible,  and  more  or  less  prominence  can  be  given 
the  shunt  or  series  windings,  as  may  be  required. 

Limit  of  Frequency.  —  While  60  cycle  rotary  converters 
of  a  capacity  as  great  as  500  K.W.  have  been  built,  the 
greatest  success,  up  to  the  present  time,  has  undoubtedly 
been  obtained  by  using  a  frequency  of  approximately  40 
cycles  and  under.  The  limit  of  frequency  of  a  rotary  con- 
verter is  due  solely  to  mechanical  reasons.  In  designing 
a  machine  for  a  given  number  of  alternations,  the  problem 
is  to  keep  the  peripheral  speed  of  the  commutator  within 
practical  limits.  Too  high  a  peripheral  speed  will  cause 
the  commutator  segments  to  buckle,  through  the  action  of 
centrifugal  force.  A  reduction  in  the  diameter  of  the 
commutator,  on  the  other  hand,  may  reduce  the  width  of 
the  segments  below  the  lowest  limit  fixed  by  experience 
with  commutator  construction  and  operation.  The  volt- 
age and  output  will  determine  the  general  dimensions  of 
the  commutator.  Take  the  case  of  a  600  K.W.,  550  volt, 
60  cycle  rotary  converter,  the  speed  of  which,  on  account 
of  its  size,  is  limited  to,  say,  600  R.P.M.  The  number  of 
poles  would  be  twelve.  The  peripheral  speed  of  the  com- 


ROTARY   CONVERTERS.  I  19 

mutator  being  limited,  the  circumference  is  at  once  fixed. 
The  average  volts  per  bar  being  also  limited,  the  total 
number  of  segments  is  determined.  In  a  40  cycle  rotary 
recently  constructed,  the  average  voltage  between  seg- 
ments was  limited  to  1 31  volts,  and  the  commutator  speed 
to  4,500  feet  per  minute.  If  we  apply  this  data  to  the  60 
cycle  rotary,  we  have  the  following  : 

Number  of  segments  between  poles  =  550-5-  13^  =41 

Total  number  of  segments  =  12  (number  of  poles)  X  41  =492 

That  circumference  of  the  commutator  which  will  keep 
the  peripheral  speed  within  the  limits  set  —  i.e.,  4,500  feet 
per  minute  —  is  90",  thus  allowing  only  j  8",  for  the  width 
of  each  segment.  For  mechanical  reasons  this  width  is 
less  than  can  be  used.  It  will  be  seen  that,  unless  the 
speed  of  the  rotary  can  be  increased,  thus  permitting  a 
lesser  number  of  poles,  or  the  peripheral  speed  of  the  com- 
mutator can  be  increased,  permitting  a  larger  circumfer- 
ence, and  consequently  wider  segments,  the  difficulty  can 
only  be  overcome  by  using  a  double  commutator.  This, 
however,  involves  a  complication  of  collector  rings  and 
connections,  and  the  current  must  be  commuted  twice 
and  the  commutator  losses  doubled.  This  rotary  could 
be  built  with  one  commutator,  if  wound  for  1 1  o  volts  or 
thereabouts.  The  general  statement  may  be  made  that, 
for  frequencies  over  35  to  40  cycles,  it  is  more  difficult  to 
build  rotaries  for  high  voltage  than  for  low  voltage,  —  i.e. 
for,  say,  550  volts,  —  than  for  100  to  200  volts,  but  not 
such  a  difficult  problem  to  wind  a  converter  of  under  35 
cycles  for  the  higher  voltage. 

Regulation  of  Voltage  by  Field  Excitation — Like  the 
synchronous  motor,  the  rotary  converter  can  be  used  to 


120       POLYPHASE    APPARATUS    AND    SYSTEMS. 

annul  self-induction  of  the  line  and  the  results  of  poor 
power  factors  of  other  parts  of  the  system.  For  purposes 
of  automatic  compounding,  the  shunt-wound  rotary  con- 
verter is  useless  on  account  of  its  constant  power  factor. 
The  compound  rotary,  however,  fulfils  the  exact  conditions 
required  for  overcoming  the  drop  in  line,  and  thereby  main- 
taining constant  voltage  at  the  direct-current  end,  or  for 
raising  the  alternating  voltage  with  increasing  load,  and, 
thereby,  the  direct-current  voltage.  This  regulation  can 
be  effected  without  any  change  in  the  generator  excitation 
simply  by  varying  the  phase  relationship  of  current  and 
volts. 

As  an  illustration  of  the  use  of  this  valuable  feature  of 
a  rotary  converter,  let  us  take  the  case  of  a  generator,  with 
constant  field  excitation,  supplying  current  to  a  converter  for 
street  railway  service,  over  transmission  lines  having  a 
reactance  and  resistance.  The  voltage  drop  is  still  further 
increased  at  full  load  by  the  reactance  of  generator  and 
converter. 

The  compound  field  of  the  rotary  is  proportioned  so  that 
at  no  load  it  is  underexcited.  The  E.M.F.  of  the  rotary  is 
then  considerably  less  than  the  impressed  E.M.F.,  and  cur- 
rent in  the  line  is  made  lagging.  The  E.M.F.  of  self- 
induction  is  thereby  increased  so  that  the  voltage  of  the 
system  is  cut  down,  giving  a  voltage  at  the  collector  rings 
corresponding  to  the  500  volts  direct  current. 

As  the  load  increases,  the  excitation  is  increased  by  the 
series  fields,  thereby  increasing  the  rotary  E.M.F. ,  and  at 
some  intermediate  point  bringing  current  and  E.M.F.  in 
phase.  The  drop  of  voltage  is  then  due  to  resistance 
only.  At  full  load  the  converter  is  overexcited,  and  the 
rotary  E.M.F.  is  greater  than  the  impressed.  The  current 


ROTARY   CONVERTERS.  121 

is  then  leading,  and  the  voltage  is  actually  higher  at  the 
converter  than  at  the  generator.  In  this  way  the  pressure 
at  the  commutator  of  the  rotary  is  made  550  volts. 

The  excitation  can  be  adjusted  so  as  to  maintain  con- 
stant voltage  at  the  commutator  brushes,  the  automatic 
regulation  taking  care  of  line  and  converter  drop  only. 

For  any  particular  over-compounding  or  compensation  of 
voltage  drop,  a  certain  amount  of  self-induction  must  be 
present  in  the  system.  The  best  results  in  compounding 
are  obtained  when  the  rotary  is  operated  from  its  own  inde- 
pendent circuit,  and  when  generator,  line,  and  converter  are 
carefully  adjusted  for  the  compounding  required.  This  ad- 
justment not  infrequently  includes  an  artificial  reactance 
such  as  a  choking  coil. 

A  graphical  demonstration  of  the  variation  of  voltage 
due  to  power  factors,  both  lagging  and  leading,  is  given  in 
Chapter  I. 

Power  Factor.  —  The  power  factor  of  the  compound- 
wound  rotary  converter  excited  for  unit  power  factor  at 
full  load  is  not  so  good  at  light  loads.  The  power  factor 
of  the  shunt-wound  converter  we  have  seen  is  the  same  at 
all  loads.  The  induction  type  has  a  variable  power  factor 
which  is  not  so  good  as  that  of  the  compound  rotary. 

A  variation  of  the  reactance  in  the  supplying  circuit 
will  change  the  curve  of  power  factor  for  various  loads. 
This  is  due  to  the  fact  that  the  field  excitation  must  be 
increased,  or  reduced,  as  the  case  may  be,  in  order  to  main- 
tain a  100  per  cent  power  factor  at  any  predetermined 
percentage  of  load.  To  obtain  10  per  cent  over-com- 
pounding, the  fields  of  the  compound  rotary  converter  are 
excited  to  give  a  power  factor  of  unity  at  usually  ^  load  ; 
and  when  it  is  desired  to  maintain  a  constant  voltage  at 


122       POLYPHASE   APPARATUS   AND    SYSTEMS. 

the  commutator,  the  fields  are  ordinarily  adjusted  to  give 
this  power  factor  at  full  load.  Mr.  E.  J.  Berg,  who  has 
given  much  study  to  the  practical  application  of  these 
principles,  has  calculated  some  power-factor  curves  which 
illustrate  the  amount  of  reactance  necessary  to  effect  a 
compounding  of  the  direct  current  in  a  rotary  converter 
to  which  current  is  supplied  by  its  own  transmission  line. 
In  Fig.  8 1,  curve  2  is  the  curve  of  power  factor  of  a  series- 
wound  rotary  when  excited  to  have  a  power  factor  of  unity 


Per  Cent. 
100 
90 

80 

-10,, 

a  =40 

\  ] 
Powt 

rF_ 

tctor  a, 

Gt 

\  — 
nerator 

r=j 

-70, 

8  = 

40 

^ 

#7T^ 

• 

—  • 

: 

-^ 

-\ 

-X 

^ 

// 

~K 

-  -^. 

••^ 

560 

Iso 

140 

-30 
20 
10 

I 

f 

~~i 

/^ 

f 

Energy  Drop  Component              =T 
Reactance  "           "                       =S 

( 
A 

fo^ 

rV 

, 

' 

111 


.1    .2    .3    .4    .5    .6    .7    .8    .9    1.01.11.21.31.41.51.61.71.81.92.0 
Output  from  Continuous  Current  Side  of  Rotary 
Fig".  81. 

at  2  load.  The  reactance  of  the  generator,  line,  and  con- 
verter is  assumed  as  40  per  cent  ;  the  resistance  as  10  per 
cent  ;  the  generator  excitation  is  also  assumed  as  constant 
under  these  conditions.  The  power  factor  at  full  load  is 
98^  per  cent ;  at  |  load,  100  per  cent  ;  -J-  load,  97. V  per 
cent  ;  i  load,  79  per  cent ;  and  at  T^  load,  47  per  cent. 
Mr.  Berg  then  assumes  that,  instead  of  constant  field  exci- 
tation of  the  generator,  the  terminal  voltage  is  kept  con- 
stant. This  would  correspond  to  a  case  where  the  rotary 
transmission  lines  were  fed  from  the  station  bus-bars.  The 
total  reactance  of  the  system  is  then  reduced  by  that  of  the 


ROTARY   CONVERTERS.  123 

generator,  becoming  10  per  cent.  Curve  3  shows  the  power 
factor  at  all  loads  for  these  conditions.  It  is  necessary  to 
reduce  the  excitation  in  order  to  maintain  the  power  factor 
unity  at  3  load.  The  power  factor  is  much  lower  at  other 
loads,  and  the  condition  of  operation  is  by  no  means  as  sat- 
isfactory as  before.  The  plant  can  be  made  so  by  introdu- 
cing in  the  line  an  external  reactance  equal  to  the  generator 
reactance.  This  may  be  any  form  of  a  choking  coil.  The 
power-factor  curves  at  the  generator  terminals,  with  the 
former  constants,  are  plotted  in  the  figure  as  curve  i. 

The  power  factors  of  an  induction  converter  of  600 
K.W.  capacity  are  as  follows  : 

Full  load .     91  per  cent 

f  load 87  per  cent 

^  load 77  per  cent 

Starting  of  Rotary  Converters.  —  Self-starting  rotary 
converters  are  set  in  operation  by  introducing  either  alter- 
nating current  to  the  collector  rings,  or  direct  current  to 
the  commutator.  When  starting  from  the  alternating-cur- 
rent end,  the  fields  should  not  be  excited.  The  starting 
current  in  a  well-designed  rotary  is  rarely  more  than  50 
per  cent  greater  than  normal  full-load  current.  This  can 
of  course  be  reduced  by  the  same  means  employed  in  the 
starting  of  synchronous  motors.  The  rotary  converter  is 
started  from  the  direct-current  end  in  the  same  way  as  a 
shunt-wound  direct-current  motor.  The  fields  should  be 
fully  excited,  and  there  should  be  a  resistance  in  series  with 
the  armature  when  the  motor  switch  is  closed.  Failure  to 
excite  the  field  may  cause  the  rotary  to  race  like  any  shunt 
motor.  Converters  are  also  frequently  started  by  auxiliary 
motors.  (See  fig.  73). 


124       POLYPHASE   APPARATUS   AND    SYSTEMS. 

Rotary  converters  can  be  rim  in  parallel  either  on  the 
direct-current  or  the  alternating-current  ends.  When  two 
or  more  rotaries  are  to  be  run  together,  they  can  be  brought 
into  synchronism  by  the  same  method  as  in  the  practice 
with  alternating-current  generators.  After  the  main  switch 
is  closed,  the  field  switch  is  then  closed,  if  the  rotary  has 
been  started  from  the  alternating-current  end.  When 
started  from  the  direct-current  end  the  machine  is  synchro- 
nized ;  then  the  field  switch,  which  supplied  excitation  for 
starting,  is  opened,  and  finally  the  switch,  supplying  its  own 
field,  is  closed. 

In  starting  a  self-exciting  or  shunt-wound  rotary  from 
the  alternating  side,  there  is  no  way  of  telling  whether  the 
polarity  will  be  positive  or  negative.  This  difficulty  may 
be  overcome  by  separately  exciting  the  machines. 

Equalizers  must  always  be  used  with  compound  type  of 
rotary  converters.  The  equalizing  switch  should  be  closed 
before  the  machines  are  thrown  together,  as  then  the  ro- 
tary will  always  maintain  the  same  polarity  at  the  commu- 
tator brushes  ;  otherwise,  if  the  series  field  predominates, 
the  current  may  be  so  far  in  advance  as  to  reverse  the 
polarity. 

Hunting The  hunting  of  rotary  converters  is  caused 

by  the  same  conditions  that  similarly  affect  synchronous 
motors.  Variation  in  the  turning  moment  of  the  prime 
movers,  short  circuits,  sudden  changes  of  load,  especially 
on  interconnecting  high  resistance  lines,  defective  design 
of  the  converters,  are  all  factors  which  affect  the  stability 
of  converters.  The  hunting  tendency  increases  with  the 
frequency.  At  25  cycles,  stability  may  be  readily. obtained. 
At  60  cycles  converters  require  careful  adjustment  to  local 
conditions  and  skillful  attention. 


STATIC   TRANSFORMERS.  125 


CHAPTER    VII. 

STATIC    TRANSFORMERS. 

Polyphase  Transformers Transformers  for  use  on 

polyphase  circuits  may  be  either  of  a  compound  type, 
wound  polyphase,  or  plain  single-phase.  Polyphase  trans- 
formers usually  have  as  many  magnetic  circuits  as  there 
are  phases,  the  flux  in  which  follows  the  same  course  as 
the  flow  of  current  in  the  corresponding  conductor  mesh. 
The  iron,  therefore,  is  used  to  better  advantage  than  in 
separate  single-phase  transformers,  and  less  is  required 
for  the  same  output.  The  two-phase  transformer  is 
sometimes  made  with  three  magnetic  circuits  and  connected 
on  the  three-wire,  two-phase  system.  Fig.  82  shows  a 
three-phase  transformer,  with  its  case  removed,  made  by 
the  Siemens-Halske  Company. 

American  engineers  for  the  most  part  use  an  appropriate 
combination  of  single-phase  transformers  for  all  the  com- 
mercial polyphase  systems.  Aside  from  the  simpler  con- 
struction and  greater  flexibility  of  the  single-phase  type, 
this  arrangement  has  the  advantage  of  not  being  rendered 
entirely  inoperative  by  damage  to  one  transformer.  The 
advantage  of  this  arrangement  is  offset  by  its  greater  cost. 
A  three-phase  three-part  transformer  can  with  proper  con- 
struction be  made  equal  to  the  single  phase  combination 
in  respect  to  continuity  of  operation. 

The  growing  application  of  electricity  to  the  transmis- 


126       POLYPHASE   APPARATUS    AND    SYSTEMS. 


sion  of  power  over  long  distances,  and  the  increasing  size 
of  electrical  units,  has  necessitated  a  change  in  transformer 
construction.     The  radiating  surface  of  a  transformer  in- 
creases as  the  square  of  its   linear  dimensions,   while   its 
mass  varies  as 
the  cube  of  the 
d  i  m  e  n  s  i  o  n  s . 
For  this  reason 
transformers  of 
a   certain  type 
of    moderate 
sizes   easily  re- 
main    cool    by 
self -radiation, 
but,  if  made  of 
greater  capaci- 
ties, would  burn 
out     unless 
cooled  by  some 
artificial  means. 
The  ordinary 
lighting    trans- 
former is  cooled 
by  being  im- 
mersed  in    oil. 
The  heat  gene- 
rat  ed  in   the 
coils    and    the 
iron  easily  finds  its  way  to  the  iron  casing,  and  is  thence 
dissipated  by  radiation.      Transformers   of   this   type  are 
rarely  built  of  larger  size  than   50  to  75   K.W.      Trans- 
formers of  greater  capacity  must  have  some  special  means 


Fig".  82. 


STATIC   TRANSFORMERS. 


127 


of  getting  rid  of  the  heat  generated  within  them.  A  num- 
ber of  methods  are  employed  for  cooling  transformers,  but 
all  may  be  classed  under  the  headings  of  self-cooled  and  of 
artificially  cooled  transformers.  It  will  be  more  satisfac- 
tory, however,  to  describe  the  various  types  under  their 
trade,  and  at  the  same  time  descriptive,  names. 

Self-Cooled  Oil  Transformers.  —  The    ordinary  lighting 

type. 


transformer  is  of  the  self-cooling 


The  magnetic  cir- 


Fig-. 83. 

cuit  is  usually  a  plain  rectangle  of  interlaced  strips  of  iron, 
permitting  a  simple  form  of  winding.  The  insulation  be- 
tween primary  and  secondary  is  tested  to  10,000  volts 
alternating.  A  twofold  advantage  is  gained  by  immersing 
these  transformers  in  oil :  First,  the  temperature  is  reduced 
by  offering  a  ready  means  of  escape  for  the  heat  ;  second, 
punctures  in  insulation  are  immediately  repaired  by  the 
inflow  of  the  oil. 

The  reduction  of  temperature  by  the  use  of  oil  is  shown 
in  Fig.  83.     Curve  I   gives   the   rise   in   temperature  of  a 


128        POLYPHASE    APPARATUS    AND    SYSTEMS. 


P 


STATIC    TRANSFORMERS.  129 

transformer  not  submerged  in  oil,  as  determined  by  the 
increase  of  resistance  method.  Curve  2  shows  the  tem- 
perature of  the  transformer  immersed  in  oil.  Curve  3  is 
the  temperature  of  the  oil.  Curve  4  is  the  temperature  of 
the  windings  of  another  transformer  of  poorer  design. 
Curve  5  shows  the  temperature  of  the  same  as  determined 
by  thermometer.  This  last  curve  does  not  give  the  true 
heating,  for  the  thermometer  cannot  reach  the  inaccessible 
portions  of  the  transformer.  These  transformers  cannot 
be  wound  for  higher  potentials  than  3,000  volts,  without 
a  serious  loss  in  capacity,  as  the  copper  must  be  sacrificed 
for  the  increased  thickness  of  insulating  material  required. 

Transformers  of  the  self-cooling  oil  type  for  high  vol- 
tages and  for  power  service  are  modified,  to  facilitate  the 
dissipation  of  the' heat  which,  in  the  larger  sizes,  could  not 
be  radiated  without  some  special  arrangement. 

Fig.  84  illustrates  a  number  of  this  type,  as  made  by  the 
Westinghouse  Electric  Company,  with  the  cases  removed. 
The  windings  are  divided  into  a  number  of  coils  which,  as 
will  be  seen,  are  spread  apart  at  the  ends,  thus  presenting 
a  large  surface  to  the  oil.  The  heat  generated  in  the  iron 
and  in  the  coils,  is  readily  communicated  to  the  oil.  The 
heated  fluid  rises,  flows  to  the  top,  and  down  the  sides  of 
the  case,  which  is  deeply  ribbed,  thus  presenting  a  large 
surface  to  the  air.  In  this  way  the  internal  heat  of  the 
transformer  finds  its  way  to  the  external  surface,  and  is 
thence  radiated  into  space. 

A  300  K.W.  transformer  of  this  type,  manufactured  by 
the  Wagner  Electric  Company,  is  shown  in  Fig.  85.  This 
transformer  is  unusually  interesting,  from  the  fact  that 
it  is  wound  for  40,000  volts,  and  is  one  of  a  number  in 
daily  use  in  the  power-house,  and  the  substation  of  the 


130       POLYPHASE   APPARATUS   AND    SYSTEMS. 

Telluride  Power  Transmission  Company.    The  transmission 
is  from  Provo  to  Mercur,  Utah,  the  distance  being  nearly 


Fig-.  85. 


forty  miles.  The  generator  current  of  700  volts  is  raised 
to  40,000  volts,  and  reduced,  at  the  receiving  end,  to  a 
suitable  voltage  for  supplying  motors  and  lights,  principally 


STATIC   TRANSFORMERS.  131 

for  mining  operations.  The  essential  features  of  the  self- 
cooled  oil  transformers,  when  designed  for  power  service, 
are  a  liberal  proportioning  of  the  mechanical  parts,  and  a 
deeply  corrugated  iron  case.  The  first  feature  produces  a 
rapid  convection  of  the  internal  heat,  and  facilitates  a  rapid 


.     pacEd   frV'V-'q-  "-**— '•"  ' —  ''-'  rrwrr-a 

1    ~ 


Pig-.  86. 

oil  circulation.  By  means  of  the  latter  feature,  the  exter- 
nal radiating  surface  is  two  or  three  times  greater  than  that 
which  a  plain  case  would  have. 

A  section  of  a  self-cooled  oil  transformer  for  40,000 
volts  is  shown  in  Fig.  86.  Liberal  spacing  between  the 
windings  and  the  laminations  permit  a  rapid  circulation 
of  the  oil. 

Water-Cooled  Oil  Transformers.  —  When  provided  with 
some  artificial  method  of  cooling  the  oil,  these  transformers 


132       POLYPHASE   APPARATUS    AND    SYSTEMS. 


STATIC   TRANSFORMERS. 


133 


are  smaller  and  cheaper  to  build  than  those  dependent  for 
cooling  upon  natural  radiation.  There  are  a  number  of 
methods  of  cooling  such  transformers  ;  one  of  these  is  by 
circulating  cold  water  in  a  worm  or  system  of  pipes  sur- 
rounding the  transformer  (Fig.  87) ;  another  method  of 

cooling  is  by  draw- 
ing off  the  oil,  cool- 
ing it,  and  pumping 
it  back,  the  opera- 
tion being  contin- 
u  o  u  s.  A  1,000 
H.P.  oil-cooled 
transformer  is  in 
daily  use  by  the 
Carbide  Manufac- 
turing Company, 
Niagara  Falls.  A 
motor,  pump,  and 
system  of  oil-tanks 
for  circulating  and 
cooling  the  oil  are 
used  to  control  the 
temperature  of  the 
transformer.  The 
oil  is  forced  upward  through  spaces  left  around  and  be- 
tween the  coils,  overflows  at  the  top,  and  passes  down 
over  the  outside  of  the  iron  laminations. 

In  still  another   transformer   the  windings    are    cooled 

by  the  circulation  of  water  in  flat,  thin  ducts,  interposed 

between    the    windings.     Due    form    of   this    transformer 

,of   low  secondary  voltage  has   flat   copper  tubes   for  the 

secondary   winding,    through   which   cold   water    is   circu- 


Fig-.  88. 


134   POLYPHASE  APPARATUS  AND  SYSTEMS. 

lated.  The  primary  windings  are  placed  between  the  sec- 
ondaries, so  that  the  water  circulation  in  the  latter  keeps 
both  windings  at  a  low  temperature. 

The  transformer  is  incased  in  a  circular  iron  tank,  with 
solid  base,  and  filled  with  oil.  The  province  of  the  oil 
is  to  cool  the  iron  laminations,  and  also  to  prevent  the 
condensation  of  moisture  from  the  air  on  the  cold  wind- 
ings. 

Another  form  of  oil  transformer  is  cooled  by  means  of 
a  water  jacket,  surrounding  the  case  containing  the  trans- 


'jfig.  89. 

former  proper.  Fig.  88  shows  a  Wagner  transformer  of 
this  type.  The  case  is  completely  channelled  by  water 
passages.  A  low  water  pressure  of  10  or  15  pounds  is 
sufficient  for  all  ordinary  requirements  of  service. 

Some  outside  source  of  power  is  required  to  operate  the 
cooling  devices,  which  slightly  reduces  the  total  efficiency 
of  the  transformation.  The  water  coil  may  be  conve- 
niently supplied  by  water  mains,  or,  in  the  case  of  a  water- 
power  transmission,  by  the  water  under  head. 


STATIC    TRANSFORMERS. 


135 


Air-Blast  Transformers.  —  In  this  transformer  the  cool- 
ing is  effected  by  means  of  a  forced  current  of  air  circulat- 
ing through  the  windings  and  core. 

The  primary  and  secondary  coils  are  separately  wound 
on  formers  and  insulated,  and  then  assembled  in  groups 
(Fig.  89),  the  coils  being  intermingled.  The  groups  are 


Pig-    9O. 

assembled  in  the  form  of  a  case,  being  separated  from  one 
another  by  vertical  air  spaces.  The  iron  case  is  then  built 
up  around  the  windings  (Fig.  90),  the  laminations  being 
horizontally  spaced  at  frequent  intervals. 

It   is   evident  that  this  construction   permits  the   most 
complete  ventilation,  as  the  very  heart  of  the  transformer 


136   POLYPHASE  APPARATUS  AND  SYSTEMS. 

is  reached  by  the  blast  of  air.  The  flow  of  air  is  con- 
trolled by  means  of  two  dampers,  one  of  which  is  located 
at  the  top  of  the  transformer,  regulating  the  air  between 
the  windings;  the  other  is  on  the  side  of  the  frame,  and 
controls  the  flow  of  air  through  the  core.  Fig.  91  shows 
the  arrangement  of  iron  and  copper  parts  and  ventilating 
ducts;  and  Fig.  92  a  completed  transformer  in  its  frame. 


Fig-.  91. 

The  apparatus  for  funishing  the  air  blast  consists  of  a 
blower,  and  is  usually  operated  by  a  motor,  the  air  being 
delivered  to  the  transformer  by  means  of  a  flue.  The 
volume  of  air  required  for  cooling  purposes  varies  with  the 
number,  size,  and  efficiency  of  the  transformers. 

The  following  table  gives  the  volume  and  pressure  of  air 
required  for  transformers  of  various  sizes,  of  the  average 
efficiency. 


STATIC    TRANSFORMERS. 


137 


X 

o  jg 

b.  <A 

K  ~ 

w 

•  W 

^  S 
^"  o 

.W.  SIZE  o 

tANSFORME 

UNITS. 

SIZE  OF 
BLOWER. 

SPEED  OF 
BLOWER. 

>  X 

s£  . 

W  N  « 
H  £  g 

IS  « 

W  W  g 

s  §^ 

z  S  ^ 

^>   X 

u.  FT.  AIR 

QUIRED  PE 
ANSFORMEI 

2 

Q  * 
I—  1  a 

°s 

HH 

*H 

D  U 

O 

o 

(j  w  x 

ffi 

300 

50 

40" 

375 

1,  800 

•30 

250 

•25 

900 

ICO 

s°" 

35° 

3,200 

.40 

35° 

.60 

1,  800 

200 

60" 

325 

5,9°o 

•5° 

600 

I.IO 

2,700 

300 

70" 

310 

8,800 

.60 

850 

2.25 

4,5°° 

500 

80" 

310 

13,000 

.80 

1,300 

4-25 

6,750 

750 

90" 

295 

17,600 

.90 

i,  800 

6-75 

7,5°° 

1,250 

100" 

280 

23,600 

I. 

3,000 

12. 

From  the  table  it  will  be  seen  that  the  power  consumed 
in  cooling  the  transformers  is  less  than  .  i  of  i  per  cent  of 
the  output  of  the  transformers.  If  the  transformers  have 
an  efficiency  of  97.5  per  cent  at  full  load,  the  total  effici- 
ency of  transformation  is  reduced  to  97.4  per  cent  by  the 
use  of  the  air  blast  —  a  perfectly  negligible  quantity. 

In  case  of  damage  to  the  cooling  arrangement,  the  trans- 
formers can  operate  for  a  few  hours  without  the  air  blast. 
It  is  desirable,  however,  to  always  provide  this  apparatus  in 
duplicate. 

The  material  protecting  the  primary  and  second  coils 
and  the  windings  from  the  case,  has  for  the  same  thickness 
a  considerably  greater  insulating  property  than  oil  or  air. 

Operation  of  Air-Blast  Transformers When  transform- 
ers of  this  type  are  run  in  groups  or  "banked"  together, 
care  should  be  take  that  the  air  enters  each  transformer 
at  the  same  pressure,  otherwise  the  transformers  will  heat 
unequally.  This  can  be  accomplished  by  having  the  flue 
from  the  blower  to  the  transformer  of  such  area  that  the 
velocity  of  the  air  will  not  exceed  200  feet  per  minute. 


138       POLYPHASE   APPARATUS   AND    SYSTEMS. 

The  most  desirable  installation  of  the  transformer  is  over 
a  closed  chamber  of  sufficient  size  to  admit  inspection  of 
the  windings.  Unequal  pressure  in  different  transformers 


Fig-.  92. 


may  be  compensated  for  by  means  of  the  two  dampers.  The 
temperature  of  the  outgoing  air  affords  a  ready  means  of  de- 
termining the  proper  amount  of  air  to  be  admitted  to  each 


STATIC   TRANSFORMERS. 


139 


transformer.  The  air  supply  is  sufficient,  if,  at  full  load, 
it  does  not  heat  more  than  20°  Centigrade  above  the  sur- 
rounding atmosphere. 

Transformers  of  different  capacities,  or  even  of  the 
same  capacity,  should  not  be  operated  in  parallel  unless 
they  have  the  same  electrical  constants  ;  otherwise,  the 
load  will  be  unequally  divided.  Parallel  connections  should 


nsfer**'*  are  d<lt*(U connects. 
Three-phase  Generator 


Fig.  93. 


have  the  least  possible  resistance  for  the  same  reason. 
Fig.  93  shows  the  installation  and  connections  of  air-blast 
transformers  in  a  long-distance  power  transmission. 

Natural-Draft  Transformers Transformers  of  this  type 

are  self-cooled  by  a  natural  circulation  of  air.  Fig.  94 
shows  a  transformer  without  its  case.  They  are  designed 
to  have  very  large  radiating  surfaces,  compared  with  their 
capacity.  As  usually  constructed,  the  windings  are  on  the 


140   POLYPHASE  APPARATUS  AND  SYSTEMS. 


outside  of  the  core,  instead  of  being  surrounded  by  it. 
Every  facility  is,  therefore,  present  for  the  radiation  of  heat 
from  the  coils.  The  transformer  is  mounted  upon  a  solid 
foundation,  and  then  covered  with  a  corrugated  sheet-iron 
cylinder,  provided  with  bottom  openings  and  a  ventilating 
roof.  This  construction  allows  a  free  and  natural  circula- 
tion of  air  through  the  casing  and  around  the  transformer. 
These  transformers  are 
built  for  10,000  volts 
or  1 5,000  volts,  and  of 
capacities  from  5  to  50 
K.W.  They  are,  as 
might  be  expected, 
more  expensive  than 
either  oil  or  air-blast 
transformers.  They 
have  the  compensating 
advantages  of  not  re- 
quiring any  artificial 
cooling  device. 

Efficiency  and  Loss- 
es.—  The  character- 
istic efficiency  curve  of 
a  well-designed  trans- 
former shows  a  high 
efficiency  at  all  but 
very  light  loads.  In 
Fig.  95  the  efficiency 
of  a  250  K.  W.,  60  mg.  94. 

cycle  transformer  does 

not  fall  below  90  per  cent  from  J  load  to  about  ^  load. 
Good    efficiency,    at   light   loads,    is   a    valuable    feature, 


STATIC   TRANSFORMERS. 


141 


epecially  in  motor  and  lighting  transformers,  where  the 
average  load  rarely  makes  a  demand  of  more  than  one- 
half  of  the  transformer  capacity.  The  efficiencies  of  the 
250  K.W.,  taken  from  the  curve,  are  as  follows  : 

T\y   load  .     .     ^ 87       per  cent 

i     load 94.6  per  cent 

•J-     load 97      per  cent 

load 97.7  per  cent 

load 98      per  cent 

load 98.1   per  cent 


I 

full 


8 


8 


X 


b 

I 

o 
$ 
Ul 

**. 
o 


/ 


3 


8 


g 


Mo 


-  £e/?f.  of  Load 
Figr.  95. 


The  losses  in  a  transformer  consist  only  of  copper  and 
iron  losses.  The  former  vary  with  the  load,  while  the  iron 
or  core  losses  remain  about  the  same  for  all  loads.  It  is 
necessary,  therefore,  in  order  to  obtain  good  efficiency  at 
light  loads,  to  reduce  the  core  losses  to  a  minimum. 
Judging  from  the  shape  of  the  curve  in  Fig.  95,  the  core 


142       POLYPHASE   APPARATUS   AND    SYSTEMS. 


loss  must  be  small,  This  is  shown  to  be  the  case  in  Fig. 
96,  which  gives  the  watts  lost  in  the  iron  of  the  same 
transformer,  and  also  the  corresponding  exciting  current. 
At  full  voltage,  the  exciting  current  is  2.3  amperes,  and  the 
core  loss  3,380  watts,  or  i^  per  cent  of  the  full-load  input 
of  the  transformer.  The  exciting  current  being  a  lagging 
current  does  not,  of  course,  represent  a  corresponding 
waste  of  energy.  The  loss  in  the  copper  conductors  is 
only  f  of  i  per  cent.  By  reducing  the  amount  of  copper 
in  both  primary  and  secondary  coil,  say,  one-half,  we  obtain  a 
proportionately  increased 
loss  in  the  copper,  or  a 
reduction  in  the  efficiency  2300 
of  transformer  from  98 
per  cent  to  approximately 
97.5  per  cent.  But  the 
total  cost  of  the  trans- 
former is  thereby  de- 
creased from  10  to  20 
per  cent. 

It  is  not  always  wise  to 
select  the  more  efficient 

transformer,  especially  in  water-power  transmissions,  where 
the  chief  item  in  the  cost  of  delivered  power  is  the  interest 
on  the  plant,  nor  in  plants  where  there  is  not  a  demand  for 
every  horse-power  developed.  As  an  illustration,  take  the 
case  of  a  power  transmission  of  1,000  H.P.  using  the 
cheaper  transformer,  which  has  an  efficiency  of  97.5  per 
cent.  The  power  delivered  is  about  I  per  cent,  or  10  H,P., 
less  than  with  the  more  efficient  step-up  and  -down  trans- 
formers. If  a  market  were  found  for  every  horse-power 
transmitted  at,  say  $30  per  H.P.  per  year,  the  loss  in  reve- 


0   .4   .8   1.2  1.6   2.0  2.4  2.8  3.2  3.6 
0   400  800  1200  1600  2000  2400  2800  3200  3600 

Fig.  96. 


STATIC    TRANSFORMERS. 


143 


nue  to  the  power  company  would  be  $300  a  year.  As  a 
partial  offset,  there  would  be  the  interest  on  the  difference 
in  the  first  cost  of  the  transformers.  Few  water-power 
transmissions,  however,  are  run  at  their  full  capacity. 
When  such  is  the  case,  the  power  company  is  usually  war- 
ranted in  buying  the  expensive  transformer.  In  the  trans- 
mission of  steam-generated  power,  fuel  is  generally  the 
most  important  single  factor  in  the  make-up  of  the  total 


I. 


?e2 
S 


02468  10 

AMPERE  PRIMARY 

Fig.  97. 

cost  of  power,  and,  as  a  rule,  the  most  efficient  transform- 
ing devices  should  be  used. 

Regulation.  —  Regulation  in  a  transformer  is  the  per- 
centage drop  of  secondary  voltage  from  no  load  to  full  load, 
the  primary  voltage  remaining  constant.  Good  regulation 
is  more  desirable  in  a  transformer  than  in  a  generator,  as 
there  is  no  means  in  the  former  apparatus  of  compounding 
for  the  voltage  drop.  On  a  non-inductive  load,  the  regula- 
tion is  equal  to  the  I.R.  drop  of  the  secondary.  On  an 
inductive  load,  the  regulation  is  the  drop  due  to  the  result- 


144   POLYPHASE  APPARATUS  AND  SYSTEMS. 

ant  of  the  ohmic  and  inductive  components  of  resistance. 
The  regulation  in  this  case  is  the  same  as  the  impedance. 
Fig.  97  shows  the  LR.  and  the  impedance  drop  of  a  250 
K.W.,  6o-cycle  transformer.  The  impedance  curve  is 
obtained  by  short-circuiting  the  secondary,  which  gives  the 
most  inductive  condition  of  operation,  and  measuring  the 
voltage  drop  from  no  load  to  full  load.  The  non-inductive 
regulation  is  seen  to  be  .7  of  i  per  cent,  and  regulation  on 
full  inductive  load  4.29  per  cent. 

Change  of  Ratio  of  Transformation.  —  Variations  in 
voltage  are  sometimes  necessary,  as  for  instance  in  the  use 
of  rotary  converters  where  the  direct  current  E.M.F. 
must  be  varied  over  a  wide  range.  This  variation  is 
usually  obtained  by  a  corresponding  variation  in  the 
E.M.F.  of  the  alternating  current.  In  such  cases  the 
transformers  are  so  constructed  that  the  ratio  of  primary 
to  secondary  terms  may  be  changed.  This  is  accomplished 
by  bringing  out  loops  from  the  windings  and  connecting 
them  to  a  regulating  dial,  so  arranged  that  the  E.M.F. 
may  be  gradually  varied  by  moving  from  one  loop  to 
another. 


STATION    EQUIPMENT   AND   APPARATUS.       145 


CHAPTER   VIII. 

STATION    EQUIPMENT    AND    GENERAL 
APPARATUS. 

Control  of  Alternating  Current  Apparatus.  —  The  rapid 
introduction  of  generators  of  extremely  large  output,  and 
the  increasing  use  of  currents  at  high  pressures,  have 
completely  revolutionized  within  the  past  few  years  the 
switchboard  equipment  of  main-  and  sub-stations.  In  large 
installations,  the  effects  of  short  circuits  would  be  most 
disastrous,  and  the  utmost  precaution  must  be  taken 
against  their  occurrence  and  also  to  ensure  the  continuity 
of  the  service.  In  large  polyphase  plants  the  factor  of 
safety  is  increased  by  a  subdivision  of  the  feeders,  busses, 
and  even  of  the  different  phases.  By  the  use  of  barriers, 
individual  brick  compartments  and  fireproof  construction, 
the  effects  of  a  short  circuit,  however  heavy,  are  localized, 
and  cannot  materially  influence  the  system  elsewhere. 
This  subdivision  of  the  channels  of  current  transmission 
has  been  made  effective  by  the  perfection  of  the  oil  cir- 
cuit-breaker and  switch.  This  device  will  be  described 
below,  as  well  as  the  general  arrangement  of  panels  re- 
quired for  the  control,  measurement  and  protection  of 
electrical  power  plants. 

Marble  is  the  material  of  which  high-tension  switch- 
boards are  made.  Slate  is  not  suitable  .for  pressure 
greater  than  600  volts,  on  account  of  its  liability  to  cur- 
rent leakage,  due  to  the  presence  of  metallic  veins.  The 


146      POLYPHASE   APPARATUS   AND    SYSTEMS. 

instruments  are  mounted  on  the  face  of  the  panel,  the 
electrical  connections,  for  the  appliances  being  made 
behind  the  panel.  Separate  panels  are  connected  elec- 
trically by  copper  tie-bars,  uniting  the  bus-bars  at  the 
back.  They  are  connected  mechanically  by  bolts  through 
the  angle-iron  frame  behind  the  board. 

In  the  selection  of  instruments  for  a  switchboard,  a  lib- 
eral excess  allowance  should  be  made.  For  instance,  the 
range  of  the  ammeters  should  exceed  the  nominal  rating 
of  the  generators  at  least  50  per  cent.  The  voltmeters 
should  have  the  same  conservative  rating.  All  switches, 
connections,  bus-bars,  and  terminals  should  be  designed  to 
carry  full-load  current  continuously  without  appreciable 
heating. 

In  station  wiring,  cables  carrying  high  potential  cur- 
rents should  be  separated  as  much  as  possible,  and  if 
unarmored  must  be  supported  by  insulators.  The  insulat- 
ing material  covering  the  cable  may  be  weakened  by  the 
production  of  ozone,  and  in  such  cases  grounded  metal 
supports  invite  short  circuits.  The  lead  armor  of  cables 
should  be  grounded,  and  where  a  cable  joins  an  overhead 
line,  lightning  arresters  should  be  installed. 

It  is  customary  to  flare  the  ends  of  armored  cables  to  a 
bell  shape.  By  so  doing,  the  surface  leakage  is  decreased, 
and  a  sudden  change  of  electrostatic  field  is  avoided  which 
would  in  time  break  clown  the  cable  insulation. 

The  flare  of  the  cable  armor  should  be  gradual  and 
without  sharp  edges.  The  bell  is  filled  with  an  insulating 
compound,  and  the  conductor  protected  by  a  mica  tube 
leading  below  the  compound. 

Switchboards  for  Power  Transmission When  step-up 

transformers  are  used,  the  switchboard  of  the  generating 


STATION    EQUIPMENT   AND   APPARATUS.       147 

station  is  divided  into  a  low-  and  a  high-tension  equipment. 
In  the  former  are  usually  included  a  panel  for  each  gene- 
rator, a  panel  for  each  exciter,  an  exciter  feeder  panel,  a 
total-load  panel  and  oil  circuit-breaker  panels  for  the  mam- 


Fig-.  98. 

pulation  of  the  bus-bars.  Sometimes  the  exciter  and 
exciter  feeder  panels  are  combined  into  one  panel.  Fig.  98 
shows  the  connections  of  the  low-tension  board  for  a 
plant  consisting  at  present  of  two  large  three-phase 
generators  having  an  initial  pressure  of  2,000  volts.  Each 


148   POLYPHASE  APPARATUS  AND  SYSTEMS. 

generator  panel  has  mounted  in  front  volt  and  ampere 
meters,  indicating  and  induction  voltmeters,  the  mechan- 
ism for  operating  a  field  rheostat  and  a  triple-pole,  double- 
throw  oil  switch,  and  also  the  synchronizing  devices.  On 
the  rear  of  the  board  are  mounted  current  and  potential 
transformers  for  the  measuring  instruments.  By  means 
of  the  oil  switches  the  generator  may  be  thrown  on  either 
set  of  bus-bars.  The  connections  and  instruments  on  the 
exciter  panels  are  plainly  shown. 

Tlu  total-load  panel  contains  recording  wattmeters  of 
capacities  equal  to  the  total  station  load,  an  ammeter,  and 
the  wiring  and  switches  for  local  power  and  lighting 
supply. 

The  oil  switches  on  the  remaining  panels  are  in  reality 
automatic  circuit  breakers,  but  can  be  hand  operated,  thus 
permitting  a  number  of  combinations  of  the  generator  and 
bus-bar  connections. 

The  high-tension  division  of  the  switchboard  for  the 
same  plant  is  shown  in  Fig.  99.  The  arrangement  of  oil 
circuit  breakers  and  switches  which  are  now  characteristic 
of  polyphase  plants  is  clearly  shown.  The  oil  switches  on 
the  transformer  panels  are  hand  operated.  The  feeder 
panels  are  provided  with  overload  relay  oil  circuit-breakers, 
which  also  can  be  hand  tripped.  The  high-tension  parts 
of  the  board  here  considered  are  constructed  for  a  line 
voltage  of  30,000. 

The  secondary  circuit  of  all  current  and  the  neutral  of 
potential  transformers  should  be  grounded.  If  the  current 
transformer  is  mounted  on  an  insulated  support,  the  frame 
of  the  transformer  should  be  grounded.  It  is  customary 
to  use  double  or  twisted  conductor  for  the  secondary  cir- 
cuits of  both  current  and  potential  transformers. 


STATION    EQUIPMENT   AND   APPARATUS.       149 

The  connections  and  arrangement  of  generator  station 
panels  for  a  typical  polyphase  railway  plant  are  given  in 
Fig.  100.  The  generators,  which  may  have  an  initial 
pressure  of  6,600  or  13,200  volts,  feed  direct  into  the 
high-tension  lines.  A  double  protection  against  short  cir- 


Fig.  99. 

cuits  consists  of  the  non-automatic  oil  switch  on  the  gene- 
rator panel  and  the  automatic  switch  on  the  line  panel. 
The  other  panels  are  an  exciter  and  an  exciter  feeder 
panel.  An  additional  panel  is  of  course  required  for  each 
additional  exciter,  and  also  for  each  additional  generator. 


150   POLYPHASE  APPARATUS  AND  SYSTEMS. 

The    sub-station    lay-out  is   shown  in    Fig.    101.      The 
high-tension  lines  are   tied  to  the  bus-bars    of  the    A.  C. 


converter  panel.  The  main  transformers  are  connected 
to  the  busses  by  automatic  oil  switches.  The  T.P.  D.T. 
switch  between  the  transformers  and  converter  is  mounted 


STATION    EQUIPMENT   AND   APPARATUS.       151 

on  a  small  panel,  and  is  used  for  starting  the  converter 
from  half-voltage  secondaries,  the  connections  of  which  are 
shown.  The  other  panels  are  the  D.C.  converter  and  D.C. 
feeder  panels,  the  connections  of,  and  instruments  belong- 
ing to,  which  are  shown  in  the  diagram. 

The  general  arrangement  of  the  switchboard  apparatus 


D.C.    FEEDER   PAIIEL 


D.C.  ROTARY  CONVERTER  PANEL 
POSITIVE  BUS 


A.C.    ROTARY   CONVERTER   PANEL 


Fig-.  101. 

for  the  control  of  the  new  Niagara  plant,  includes  an  oper- 
ating board,  feeder  and  generator  switches,  two  sets  of 
bus-bars,  and  recording  wattmeter  panels.  The  operating 
board  is  placed  upon  a  gallery  located  centrally  with  respect 
to,  the  power-house  floor. 


152   POLYPHASE  APPARATUS  AND  SYSTEMS. 

Oil  switches  are  placed  in  two  lines,  back  to  back,  lo- 
cated on  the  floor  centrally  with  respect  to  the  group  of 
six  .alternators. 

The  bus-bars  are  located  in  a  cable  subway  under  the 
line  of  oil  switches.  Recording  feeder  and  wattmeter 
panels  are  placed  in  the  office  of  the  Superintendent. 

The  operating  board  at  present  consists  of  12  feeder 
panels,  one  exciter  panel,  one  inter-connected  and  six  gen- 
erator panels.  The  generating  panels  contain,  in  addition 
to  the  usual  measuring  and  indicating  instruments,  one 
generator  oil  switch  with  overload  time  limit  relay,  and  two 
generator  selecter  oil  switches. 

The  feeder  panels  contain  measuring  and  indicating  in- 
struments, and  also  two  feeder  oil  switches  with  time  limit 
relay. 

The  oil  break  switches  for  generator  and  feeder  control 
are  of  the  four-pole,  single-throw,  oil-break  type,  electrically 
operated.  Each  switch  is  capable  of  carrying  3750  K.W. 
at  2,200  volts,  two-phase,  without  excessive  heating,  and 
can  break  the  circuit  at  this  load.  The  two  sets  of  bus- 
bars are  placed  on  insulating  supports  in  fire-proof  com- 
partments. 

High-Tension  Switches  and  Circuit  Breakers.  —  The 
necessity  of  at  times  disconnecting  transmission  lines  from 
sources  of  electric  power,  both  under  load  and  on  short 
circuit,  in  stations  of  large  capacity,  has  caused  to  be 
developed  various  types  of  switching  mechanism.  The 
best-known  types  of  switches  for  the  purpose  are,  first, 
switches  which  break  the  circuit  in  the  open  air  ;  second, 
switches  which  break  the  circuit  in  a  confined  air  space  ; 
third,  switches  which  break  the  circuit  in  oil. 

Fig.  1 02  gives  the  outline  of  a  typical   open-air    switch. 


STATION    EQUIPMENT   AND   APPARATUS.       153 


The  one  shown  is  designed  for  interrupting  a  circuit  of 
100  amperes  and  20,000  volts.  Tests  have  demonstrated 
that  the  open-air  switch  is  not  suited  as  a  circuit  breaker 
under  load.  The  arc  sometimes  holds,  and  if  the  head 
room  is  insufficient  may  give  rise  to  trouble.  On  ex- 
tremely high-voltage  circuits  the  switch  should  not  be 


MARBLE  BARRIER 


SWITH  BLADE 


Fig.  102. 

used,   as    at    40,000   volts    the    arc    has    been   known    to 
flare  up  from  the  goat  horns  to  a  very  considerable  dis- 


tance. 


One  form  of  the  expulsion  or  inclosed-air  switch  is 
shown  in  the  cut,  Fig.  103.  This  is  a  three-phase  switch 
with  a  double  break  in  each  phase.  The  U-shaped  rods 


154      POLYPHASE   APPARATUS   AND   SYSTEMS. 


make  contact  at  the  top  clamps.  The  circuit  may  be 
automatically  or  hand  tripped  by  suitable  mechanism  on 
the  back  of  the  board.  The  break  takes  place  in  the  tubes, 
which  are  of  fiber.  The  rods  are  forced  down  at  great 
speed  by  the  gases  of  the  arc,  and  caught  by  the  dash-pot. 
Within  its  capacity,  this  switch  is  much  more  reliable 

than  the  open-air  type.      It 
h=^  w^  s  not  adapted  ^or  extremely 

!    f"  |i     high  voltages.     It  has  been 

used  with    success    on    cir- 
cuits of  25,000  volts. 

Switches  which  break  the 
circuit  in  oil    are   now  em- 

|i,  I   |i.      ployed    in    this   country   al- 

HIUI  i,  most  to  the  exclusion  of 
other  types.  When  prop- 
erly constructed,  they  are 
unfailing  in  their  action, 
even  on  short  circuits  at 
potentials  as  high  as  40,000 
volts.  The  switch  in  its 
simplest  form  consists  of 
one  or  more  sets  of  con- 
tacts which  are  made  or 
broken  under  oil  contained 
in  a  surrounding  vessel. 
A  typical  three-pole  oil- 
break  switch,  with  oil  tank  removed,  is  shown  in  Fig.  104. 
Each  pole  is  seen  to  have  two  breaks.  This  switch  has 
opened,  with  entire  success,  circuits  of  1300  K.  V.  A.  at 
25,000  volts. 

For  generating  and  sub-stations  of  very  large  capacity, 


Fig.  1O3. 


STATION   EQUIPMENT   AND   APPARATUS.       155 

the  oil  switch  is  constructed  to  break  each  phase  in  a 
separate  compartment  or  cell.  A  two-phase  switch  of 
this  form  consists  of  four  single-phase  double-pole  switches 
or  elements,  and  a  three-phase  consists  of  three  such  ele- 


Fig.  104. 

ments, — all,  however,  making  or  breaking  contact  simul- 
taneously. In  addition  to  the  single-phase  elements  which 
are  supported  on  a  platform,  the  switch  is  provided  with  a 
top  operating  mechanism. 


156      POLYPHASE   APPARATUS    AND    SYSTEMS. 

The  general  view  of  an  electrically  operated  switch  of 
this  form,  with  brick  partitions  removed,  is  shown  in  Fig. 
105.  The  internal  connections  of  a  single  element  are 
shown  in  outline  in  Fig.  106. 

Each  single-phase  element  consists  of  two  metal   cylin- 


Fig.  1O5. 

ders  which  contain  the  oil  and  contacts.  The  incoming 
lead  is  attached  to  one  cylinder,  and  the  outgoing  of  the 
same  phase  to  the  other.  Two  copper  rods  joined  by  a 
metallic  cross-head  slide  through  an  insulating  sleeve,  and 
make  contact  at  the  bottom  of  the  cylinders  when  closing 


STATION    EQUIPMENT   AND    APPARATUS.       157 


tjJJJ] 


the  circuit.      The  switch  may  be   operated   by  compressed 

air  or  by  a  motor.      When  electrically  operated,  the  motor 

drives  a  worm  gear,  which  transmits  the  motion  to  a  crank 

and  cross-head,  through  a  friction  clutch.     At  both  ends  of 

the   stroke,  the   cross4iead   compresses  a  spring  which,  as 

soon  as  the  crank  passes  the  center  line,  instantly  throws 

the    cross-head     to    an 

open  or  closed  position, 

depending  upon  whether 

the    switch  was    at   the 

time    open     or     closed. 

The  cross-head  and  con- 

tact rods  are  connected 

together  by  wooden  rods 

which    provide    for    the 

mechanical      movement 

of  the  contacts,  and  also 

thoroughly  insulate  the 

different  poles.   The  mo- 

tor    is      controlled    by 

means  of   a  single-pole, 

double-t  h  r  o  w,    h  a  n  d-    ,- 

operated    switch,     o  n  e  L 

throw     being    used     to 

open    the    switch,     the 

other  to  close  it.     There  Fig-  1O6' 

is  also  an  automatic  switch  on  the  top  mechanism,  whose 

function  is  to  automatically  open  the  motor  circuit.     When 

the  hand-operated  switch  is  thrown,  the  motor  starts,  and 

runs  until  the  crank  rotates  to  a  nearly  vertical  position  ; 

then    the  automatic   switch   opens  the  motor  circuit,   and 

lights  a  lamp  which  informs  the  switchboard  attendant  that 


158   POLYPHASE  APPARATUS  AND  SYSTEMS. 

the  switch  has  operated.  The  three  phases  are  broken  or 
closed  simultaneously.  \Yhen  the  three  sets  of  U-shaped 
conductors  are  lifted,  the  circuit  is  broken  under  oil  at  two 
points  in  each  phase,  or  at  six  points  in  each  three-phase 
switch.  The  movement  of  the  rods  -is  17  inches  in  the 
larger  form  of  this  switch. 

By  separating  each  element  of  the  switch  by  brick  and 
soapstone  partitions,  a  burn-out  in  one  cell  cannot  be  com- 
municated to  an  adjacent  cell. 

Careful  selection  of  a  suitable  grade  of  oil  should  be 
made.  A  light,  clear,  mineral  oil  of  low  flashing  point, 
supplied  by  the  switch  manufacturers  alone,  should  be  used. 

Renewal  of  the  oil,  of  course,  is  necessary,  depending  on 
the  extent  of  the  carbonizing.  No  fixed  rule  can  be  given. 
In  general,  new  oil  is  not  required  oftener  than  once  in 
three  months,  and  in  most  cases  not  oftener  than  once  a 
year,  where  the  switch  works  on  circuits  of  10,000  or  lower 
volts. 

On  connecting  a  source  of  electrical  power  to  a  line  con- 
taining inductance  and  capacity,  an  oscillation  is  set  up 
in  the  system.  This  will  be  the  case  irrespective  of  the 
type  of  connecting  device.  On  disconnecting  the  genera- 
tor on  the  line  from  its  load,  the  arc  will  hold  for  a  number 
of  half  waves.  It  has  been  found  that  with  air  switches 
of  all  types,  the  break  is  accompanied  by  oscillating  cur- 
rents which  may  be  of  excessive  voltage,  endangering  the 
insulation  of  line  and  apparatus.  A  short  circuit  in  a 
system  where  considerable  energy  may  be  stored,  due  to 
the  inductance  and  capacity,  when  interrupted  by  an  air 
switch  may  produce  destructive  effects. 

The  oil  switch  on  account  of  the  gradual  break  permits 
the  stored  energy  to  discharge  itself  without  high-voltage 


STATION    EQUIPMENT   AND   APPARATUS.       159 

oscillations.  This  is  an  invaluable  feature,  and,  with  the 
absolute  safety  and  effectiveness  of  the  device,  marks  the 
oil  switch  as  one  of  the  most  important  developments  in 
the  art  of  recent  years. 

The  various  types  of  switches  described  above  may  be 
either  operated  by  the  hand  of  the  attendant  or  provided 
with  suitable  mechanism  for  automatic  operation. 

When  the  tripping  mechanism  becomes  an  integral  part 
of  the  switch,  it  has  been  proposed  to  call  the  switch  a 
circuit  breaker.  This  is  really  a  distinction  without  much 
difference.  All  the  switches  when  automatically  arranged 
may  be  considered  as  coming  within  the  meaning  of  the 
expression  "circuit  breakers." 

The  fuse  has  been  practically  discarded  in  favor  of  the 
automatic  switch  for  extremely  high-tension  work  of  large 
magnitude. 

For  the  protection  of  generators  and  translating  appa- 
ratus against  destruction  under  a  heavy  overload  or  a  short 
circuit  disconnecting  devices  must  be  made  to  operate 
automatically. 

The  types  of  switches  already  described,  especially  the 
oil  type,  can  be  made  into  automatic  circuit  breakers  by 
the  addition  of  magnetically  controlled  triggers,  which  can 
be  tripped  by  what  are  known  as  relays. 

The  relays  may  be  designed  to  immediately  respond  to 
an  overload  or  short  circuit ;  the  same  as  a  fuse  or  to  oper- 
ate the  switch  only  after  a  predetermined  and  adjustable 
length  of  time  from  the  beginning  of  the  overload,  the 
switch  remaining  unaffected,  if  the  overload  continues  for 
a  less  interval  than  this  predetermined  time. 

A  form  of  relay  is  also  used  which  acts  only  when  the  cur- 
rent enters  the  station  in  the  wrong  direction  over  the  line. 


160   POLYPHASE  APPARATUS  AND  SYSTEMS. 

Lightning  Protection.  —  There  is  no  problem  with  which 
the  electrical  engineer  has  to  deal,  that  presents  greater 
difficulties  in  the  way  of  a  positive  solution,  than  that  of 
lightning  protection.  The  uncertainty  among  the  highest 
authorities  as  to  the  exact  nature  of  lightning  phenomena 
is  partly  accountable  for  this  state  of  affairs.  The  oscilla- 
tory character  of  a  direct  lightning  stroke  has  been  estab- 
lished beyond  a  doubt,  but  experience  with  lightning  effects 
would  indicate  that  some  of  the  disruptive  discharges  act 
mainly  in  one  direction.  For  this  reason  no  one  single 
device  can  be  depended  on  to  protect  electrical  apparatus 
from  all  kinds  of  lightning  phenomena.  In  other  words, , 
there  is  not,  and  cannot  be,  a  universal  lightning  arrester. 
The  discharge  current  from  any  system  of  conductors,  pro- 
duced by  the  various  phenomena,  can  be  described  under 
three  general  heads  : 

ist.  --The  direct  discharge  due  to  the  transmission  lines 
being  in  the  direct  path  of  the  lightning  stroke. 

2d. — The  cumulative  discharge,  due  to  a  gradual  and 
sometimes  enormous  rise  of  potential  from  a  changing 
electrostatic  condition  of  the  atmosphere. 

3d. --The  secondary  discharge  due  to  secondary  cur- 
rents induced  in  the  lines  by  parallel  lightning  strokes. 

There  are  other  kinds  of  lightning  discharges  from 
transmission  lines,  which  partake,  more  or  less,  of  the 
character  of  the  above,  but  do  not  differ  greatly  in  their 
effects. 

Provision  for  the  protection  of  the  station  apparatus 
should  be  made,  not  only  in  the  station  itself,  but  along 
the  transmission  lines  as  well.  The  means  usually  em- 
ployed for  protecting  the  lines,  consist  of  guard  wires  out- 
side of  the  conductors,  or  even  of  one  guard  wire  strung 


STATION    EQUIPMENT   AND   APPARATUS.       l6l 

at  the  top  of  the  pole.  It  is  better  to  ground  this  wire  at 
every  fourth  or  fifth  pole.  In  long-distance  transmissions, 
it  is  also  well  to  install,  every  ten  miles  or  so,  line  arrest- 
ers, similar  to  those  used  in  the  station.  The  guard  wires 
protect  the  conductors  from  the  direct  lightning  stroke,  by 
discharge  to  the  ground.  They  also  have  a  dampening 


Fig.  1O7. 

effect  on  the  secondary  induced  currents,  and  those  due  to 
a  change  of  the  electrostatic  equilibrium. 

Commercial  Lightning,  Arresters.  —  From  the  foregoing, 
it  will  be  understood  that  there  is  a  good  deal  yet  to  be 
learned  about  the  most  suitable  form  of  lightning  protec- 
tion for  alternating  current  apparatus.  Nevertheless,  ex- 
perience has  narrowed  down  the  many  ancient  devices  to 
one  type  of  arrester,  i.e.,  an  arrester  composed  of  a  number 
of  metal  balls  or  cylinders,  separated  by  short  air  gaps. 


162    POLYPHASE  APPARATUS  AND  SYSTEMS. 


Fig.  107  shows  one  of  this  class,  devised  by  Mr.  Wurts  of 
the  Westinghouse  Company.  It  is  seen  to  consist  of  seven 
cylinders,  each  one  inch  in  diameter  and  three  inches  long, 
and  separated  by  spaces  ^  of  an  inch.  The  particular 
arrester  shown  is  of  the  double-pole  type,  and  designed  for 
alternating  circuits  of  1,000  volts.  When  the  discharge 

takes  place  simulta- 
neously from  two  sepa- 
r  a  t  e  conductors,  a 
short  circuit  would  fol- 
low, if  the  arc  were  not 
immediately  interrupt- 
ed. It  is  found,  with 
the  arrester  described, 
that  the  flash  is  instan- 
taneous, and  is  not  fol- 
lowed by  an  arc.  The 
passage  of  the  static 
discharge  through  the 
arrester  is  evidenced  by 
burns  or  pit-marks  on 
the  cylinder  surfaces. 
These  can  be  rotated 
on  their  axes,  in  order 
to  bring  fresh  surfaces 
opposite  each  other. 
Another  form  of  this  lightning  arrester  is  shown  in 
Fig.  108.  This  device,  made  by  the  General  Electric 
Company  consists  of  a  combination  of  short  metal  cyl- 
inders and  a  graphite  resistance.  The  single-pole  arrester 
for  1,000  volts  has  one  spark  gap  of  ^  of  an  inch, 
separating  two  metal  cylinders  two  inches  in  diameter 


Fig-.  108. 


STATION   EQUIPMENT   AND   APPARATUS.        163 

and  two  inches  long.  A  non-inductive  graphite  resistance 
is  placed  in  series  with  the  ground  wire.  The  2,ooo-volt 
single-pole  arrester  has  three  cylinders  and  two  air  gaps 
.of  approximately  63^  of  an  inch  each,  and  a  graphite 
resistance. 

The  arrester  first  described  is  made  of  an  alloy  of  zinc 
and  antimony,  and  will  operate  with  better-  results  than 
when  the  cylinders  are  made  of  copper.  The  last-de- 
scribed arresters  have  bronze  cylinders.  The  arc-extin- 
guishing action  of  these  arresters  is  dependent  mainly 
upon  the  cooling  effect  of  large  metal  masses,  and  not 
materially  upon  the  kind  of  metal.  This  cooling  effect  is 
increased  by  the  introduction  of  the  non-inductive  resist- 
ance, which,  even  in  the  event  of  the  formation  of  an  arc 
of  short  circuit,  would  materially  limit  the  volume  of  cur- 
rent. The  reversal  of  the  alternating  current  itself  extin- 
guishes die  arc,  in  the  absence  of  vapor,  which  cannot 
arise  from  the  chilled  metal  surfaces.  This  is  proved  by 
the  fact  that  a  lightning  arrester,  which  will  not  short- 
circuit  on  3,400  volts  alternating,  will  hold  an  arc  on  500 
volts  direct  current. 

Installing  Lightning  Arresters. — The  principle  on  which 
a  lightning  arrester  is  selected  for  any  particular  voltage 
is,  that  it  must  be  the  weakest  spot  in  the  line.  The  volt- 
age required  to  jump  all  the  air  spaces  should  be 'less  than 
that  which  will  puncture  the  insulation  of  the  apparatus  to 
be  protected.  The  proper  number  of  gaps  for  different 
voltages  can  be  determined  by  experiment  with  plants 
in  actual  operation.  It  has  been  ascertained  by  tests 
at  Niagara  that,  for  n,ooo  volts,  14  air  gaps  of  -gL,  inch 
in  connection  with  carbon  resistances,  afford  full  protec- 
tion with  a  margin  of  safety.  Circuits  above  2,000  volts 


164       POLYPHASE   APPARATUS   AND    SYSTEMS. 

are  protected  by  standard  2,ooo-volt  arresters  placed  in 
series. 

Fig.  109  shows  the  connections  and  method  of  installing 
the  G.E.  arrester  for  io,ooo-volt  circuits. 

Lightning  arresters  for  potentials  above  20,000  volts 
should  be  divided  into  two  or  more  sections  connected  in 
series,  each  section  consisting  of  a  small  number  of  units 


7/  Groun.d 


Fig.  109. 

well  insulated  from  the  wall.     Although  this  arrangement 
requires  a  little  more  space,  it  affords  additional  security. 

The  oscillatory  character  of  the  lightning  discharge 
gives  rise  to  great  self-induction  in  the  circuit.  It  would 
seem  as  if  a  choking  coil  placed  between  the  arrester  and 
the  electrical  apparatus  would  offer  such  resistance  to  the 
discharge  as  to  force  it,  under  all  conditions,  through  the 
arrester,  and  thence  to  the  ground.  In  actual  service  it 


STATION    EQUIPMENT   AND   APPARATUS        165 

has  been  found  that  the  choking  coil  does  not  always  offer 
this  resistance,  and  for  this  reason  its  usefulness  has  been 
questioned. 

The  uncertainty  of  action  of  one  choking  coil  in  the 
circuit  is  no  proof  of  its  inefficiency.  There  is  good  reason 
to  believe  that  this  oscillatory  discharge  has  a  wave-like 
motion,  with  maxima  and  minima  points.  If  the  arrester 
happens  to  be  placed  at  a  point  of  interference  —  a  nodal 


Pig-,  no. 

point  —  the  coil  cannot  force  the  discharge  through  the 
arrester.  This  difficulty  may  be  overcome  by  the  use  of  a 
series  of  coils  and  arresters,  arranged  as  illustrated  in  Fig. 
no.  This  shows  one  end  of  a  2,ooo-volt,  three-phase 
system  of  conductors.  The  choking-coil  may  be  made  by 
winding  150  feet  of  the  line  wire  into  a  coil,  the  inside 
diameter  of  which  is  not  less  than  i  5  inches. 


166       POLYPHASE   APPARATUS    AND    SYSTEMS. 


The  grounding  of  lightning  arresters  must  be  most  care- 
fully made,  as  upon  attention  to  this  depends  the  reliability 


Fig.  111. 


of   the    working    of    the    arresters.      The    connections   to 
ground  and  line  should  be  made  by  short,  straight  wires 


STATION   EQUIPMENT   AND   APPARATUS.       167 

of  not  less  than  No.  4  size.  A  metal  plate,  or  long  pipe, 
should  serve  as  the  ground  terminal,  embedded  in  coke,  or 
sunk  in  damp  ground  if  possible.  Fig.  1 1 1  illustrates  a 
very  effective  method  of  grounding  a  line  arrester.  It  is 
better  to  use  a  ground  plate  with  station  arresters. 

Synchronizing  Devices.  --  The  ordinary  method  of  de- 
termining whether  two  alternating  generators  are  in  par- 
allel, is  by  the  use  of  two  transformers  and  lamps,  as 
described  in  Chapter  III.  This  is  an  excellent  and  effec- 
tive method,  and  is  reliable  under  almost  all  conditions.  A 
special  device,  called  the  acoustic  synchronizer,  is  some- 
times used.  This  consists  of  two  electro-magnets,  actuat- 
ing two  inclosed  diaphragms  by  currents  from  the  machines 
to  be  synchronized.  When  the  generators  are  out  of 
phase,  the  instrument  gives  out  a  loud  pulsating  note, 
which  grows  feebler  as  synchronism  is  approached.  The 
acoustic  synchronizer,  while  accurate,  is  not  vigorous  in  its 
action,  especially  in  noisy  stations  and  on  circuits  of  low 
frequency.  An  instrument  called  the  synchronoscope  has 
been  developed  for  low-frequency  circuits,  by  the  Westing- 
house  Company.  This  apparatus  is  similar  in  appearance 
to  a  round-dial  voltmeter  or  ammeter.  When  no  current 
flows  between  the  machines,  the  needle  stands  at  zero. 
When  there  is  a  phase  difference,  a  slight  current  flows 
around  a  magnet,  which  deflects  the  needle. 

Insulators.  —  On  transmission  lines,  conveying  currents 
at  potentials  of  10,000  volts,  or  thereabouts,  and  over,  it  is 
common  practice  to  employ  porcelain  insulators.  Glass 
is  generally  used,  and  has  been  found  most  satisfactory  as 
an  insulating  line  material  for  potentials  lower  than  10,000 
volts. 

Line  insulators  for  heavy  service,  such  as  high-tension 


168   POLYPHASE  APPARATUS  AND  SYSTEMS. 

transmission  of  power,  should,  in  an  eminent  degree,  pos- 
sess two  qualities  : 

i  st.  —  Thorough  insulation  under  all  conditions  of  operation. 
2cl.  —  Great  mechanical  strength. 

When  formed  into  large  masses,  porcelain  is  supposed 
to  be  superior  to  glass  in  both  these  respects.  Glass  is 
an  almost  absolute  non-conductor  of  electricity,  but  is  said 
to  be  hygroscopic,  i.e.,  condenses  water  on  its  surface  from 
the  atmosphere,  and  thus  allows  a  leakage  of  the  current. 
When  massive,  glass  is  somewhat  difficult  to  anneal,  and 
hence  is  not  always  as  strong  mechanically  as  desirable. 

Porcelain  for  insulators  should  be  thoroughly  vitrified 
and  homogeneous.  The  material  should  be  absolutely 
non-absorbent  of  moisture,  and  sufficient  to  insulate  the 
line  even  without  the  surface  glazing.  Poor  porcelain  can 
easily  be  detected  by  the  appearance  of  the  fracture,  and 
its  porous  quality  by  soaking  in  red  ink.  Well-vitrified 
porcelain  will  show  no  signs  of  ink  when  washed ;  the 
poor  material  will  readily  absorb  it.  An  inch  thickness  of 
porous  porcelain  will  be  punctured  by  10,000  volts,  while 
the  same  thickness  of  vitrified  material  has  failed  to 
break  down  under  a  pressure  of  over  100,000  volts.  Only 
the  general  character  of  porcelain  insulators  can  be  de- 
termined in  this  rough  manner.  To  determine  the  actual 
insulating  strength,  each  insulator  should  be  submitted  to 
a  high  potential  test. 

This  test  is  best  made  by  placing  a  number  of  insulators 
inverted  in  a  metal  pan,  filled  with  a  brine  solution  to  the 
depth  of  two  inches.  The  brine  also  fills  the  pin-holes.  In 
each  pin-hole  is  placed  a  metal  rod.  All  the  rods  are  con- 
nected to  one  terminal  of  a  high-potential  circuit,  and  the 


STATION    EQUIPMENT   AND   APPARATUS.        169 


bright 
high-tension 


pan  to  the  other.  The  testing  pressure  used  is  generally 
about  40,000  volts  for  25,000  volts  service.  When  the 
circuit  is  closed,  the  defective  insulators  are  punctured,  and 
are  manifested  by  a  shower  of 
sparks.  Fig.  1 1 2  shows  a 
porcelain  insulator 
mounted  on  its  iron  pier  by 
cement.  This  insulator  is  tested 
at  60,000  volts,  and  is  used  on  a 
3O,ooovolt  transmission  in  India. 
It  is  8i  inches  in  height,  6|  inches 
in  diameter,  and  weighs  7  pounds. 

While  porcelain  is  a  superior 
material  for  insulators,  its  much 
greater  cost  than  glass  is  a  serious 
drawback.  Experience  in  their 
manufacture  has  largely  overcome 
the  difficulty  of  properly  annealing 
large  glass  insulators.  In  dry  cli- 
mates, the  hygroscopic  property 
of  glass  is  practically  nil. 

Under  such  conditions  it  would 
seem  as  if  glass  insulators  would 
be  entirely  satisfactory,  even  for  Fig>>  112' 

the  highest  voltages  that  may  be  commercially  employed. 
They  have  been  successfully  used  for  very  high  vol- 
tages, —  notably  in  the  Provo  transmission,  which  em- 
ploys 40,000  volts.  The  insulator  known  as  the  Provo 
type  is  illustrated  in  Fig.  113.  It  is  a  triple  petticoat 
insulator,  having  a  diameter  of  7  inches  and  a  height  of 
6  inches.  It  weighs  4  Ibs.  7  oz. 

A  novel  insulator,  embodying  the  insulating  properties 


1 70      POLYPHASE    APPARATUS    AND    SYSTEMS 


Fig.  113. 


STATION    EQUIPMENT   AND   APPARATUS.       i;i 

of  glass  and  the  non-hygroscopic  property  of  porcelain,  has 
recently  been  brought  out.  This  insulator  consists  of  a 
porcelain  body  and  an  inner  glass  sheath,  containing  the 
screw  pin-hole.  Oil  insulators  for  transmission  lines  have 
entirely  dropped  out  of  use  in  the  United  States. 

Pressure  Regulators One  form  of  regulator,  for  vary- 
ing the  pressure  of  an  alternating  current  circuit,  may  be 
likened  to  an  induction  motor  with  its  armature  blocked 
so  as  to  remain  stationary,  but  which  at  the  same  time  is 
capable  of  being  placed  in  various  positions,  thereby  chan- 
ging the  mutual  induction  of  the  coils.  As  ordinarily  con- 
structed, the  regulator  consists  of  what  corresponds  to  the 
field  and  to  the  armature  of  an  induction  motor.  A  hol- 
low cylindrical  structure  built  of  laminated  iron  is  provided, 
on  its  interior  surface,  with  four  slots,  in  which  are  placed 
two  coils  at  right  angles  to  each  other.  Inside  of  these 
coils  a  movable  laminated  core  is  placed,  in  such  a  manner 
that  its  position  can  be  changed  with  respect  to  the  field. 
The  winding  of  the  primary  or  field  is  connected  across 
the  lines.  While  in  the  induction  motor,  the  armature,  or 
secondary  winding,  is  short-circuited  upon  itself,  in  the  reg- 
ulator the  armature  or  secondary  winding  is  connected  in 
series  with  the  circuit  so  as  to  add  its  voltage  to,  or  sub- 
tract it  from,  that  of  the  line,  according  to  its  relative 
position  in  regard  to  the  primary  winding.  Since  the  reg- 
ulator has  some  self-induction,  and  requires  magnetizing 
current,  the  maximum  possible  boosting  obtainable  is  about 
10  per  cent  less  than  the  minimum  reducing  effect.  The 
arrangement  and  the  connections  of  this  regulator  can  be 
seen  in  diagram  Fig.  114. 

Another  type  of  regulator,  built  on  the  same  lines,  is 
sometimes  constructed  to  take  care  of  all  the  branches  of 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


a  two-  or  three-phase  circuit.  The  regulator  described 
enables  the  voltage  of  a  circuit  to  be  raised  and  lowered 
without  change  of  connections,  and  adjustments  of  pres- 
sure are  obtained  by  imperceptible  degrees. 

The  Stillwell  regulator  is  another  form  of  apparatus  for 


Fig.  114. 

raising  or  lowering  the  pressure  in  feeder  wires.  It  is  a 
transformer,  the  primary  of  which  is  connected  across  the 
circuit  and  the  secondary  of  which  is  in  series  with  the 
feeder  whose  voltage  is  to  be  regulated.  The  cut  (Fig. 
115)  shows  the  internal  connections  of  this  regulator. 
The  secondary  is  divided  into  a  number  of  coils,  which  can 
be  inserted  or  removed  from  the  circuit,  according  to  the 


STATION    EQUIPMENT    AND   APPARATUS.       173 

amount  of  variation  of  voltage  desired.  A  reversing 
switch  is  provided,  so  that  the  E.M.F.  generated  in  the 
regulator  can  be  either  added  to  or  subtracted  from  the 
pressure  of  the  feeder. 

Some  of  the  important  uses  to  which  pressure  regulators 


Fig-.  115. 

can  be  put  are  the  following  :  For  regulating  the  voltage 
of  alternating-current  feeders  ;  for  equalizing  voltage  on 
unbalanced  polyphase  circuits ;  as  dimmers  for  theaters ; 


174   POLYPHASE  APPARATUS  AND  SYSTEMS. 

as  regulators  for  series-alternating  circuits,  either  for  arc 
or  incandescent  lights. 

The  rating  in  watts  is  the  product  of  the  secondary 
current  in  amperes,  by  the  boosting  capacity  in  volts. 

Automatic  Regulators.  —  Automatic  regulators  are  em- 
ployed for  the  regulation  of  both  the  dynamo  potential  and 
that  of  separate  feeders.  The  scope  of  these  two  methods  is 
quite  different.  In  some  cases  one  would  be  desirable  and  in 
other  cases  the  other,  while  in  many  cases  it  might  be  desir- 
able to  use  both.  Both  methods  have  been  developed  to  a 
state  where  the  correction  for  variation  of  voltage  is  very 
quick,  and  by  their  use  excellent  lighting  service  is  being- 
obtained  from  systems  which  would  otheiwise  be  quite  irreg- 
ular through  variations  of  speed  or  load.  In  seme  plants 
the  voltage  of  the  generating  station  is  held  constant  by  an 
automatic  field  regulator,  and  variations  in  different  lines 
are  taken  care  of  by  hand-feeder  regulators,  while  in  other 
cases  the  voltage  at  the  station  is  allowed  to  vary  through 
its  natural  range  by  changes  of  load,  and  the  regulation  on 
different  feeders  is  taken  care  of  by  automatic  feeder 
regulators.  If  the  speed  is  reasonably  steady  they  will 
give  much  quicker  and  better  automatic  regulation  than 
can  possibly  be  obtained  by  any  other  means.  In  water- 
driven  plants,  or  other  cases  where  speed  variations  are 
troublesome,  it  may  be  desirable  to  use  automatic  field 
regulators.  The  automatic  feeder  regulator  is  intended  to 
take  care  of  variations  on  individual  feeders,  and  is  con- 
sequently useful  in  stations  whether  the  bus-bar  voltage  is 
steady  or  not.  It  can  do  the  work  of  adjustment  better 
and  more  quickly  than  any  station  attendant,  and  can  be 
made  to  operate  in  connection  with  a  compensating  device 
by  which  a  desired  pressure  at  the  end  of  the  feeder  will 


STATION    EQUIPMENT   AND   APPARATUS.       1/5 

be  automatically  maintained.  By  the  use  of  such  regula- 
tors good  lighting  service  can  be  obtained  from  circuits 
carrying  railway  load  or  which  are  for  other  reasons  sub- 
jected to  variations  which  without  regulators  would  be 
troublesome. 

Compensators  for  Alternating-Current  Circuits This 

instrument  permits  the  adjustment  of  the  potential  of 
a  circuit  at  the  station,  so  that,  notwithstanding  variations 
in  load  and  consequent  loss  or  drop  in  the  line,  the  potential 
at  the  terminals  of  the  consuming  apparatus  remains  con- 
stant. The  action  of  the  compensator  is  to  reduce  the 
indication  of  the  voltmeter  in  proportion  to  the  main  cur- 
rent. By  proper  adjustment  this  reduction  can  be  made 
equal  to  the  drop  in  the  circuit. 

In  principle  and  construction  the  compensator  is  a  trans- 
former which  has  an  adjustable  number  of  turns  which 
is  connected  in  series  with  the  supply  circuit.  The  secon- 
dary also  of  an  adjustable  number  of  turns  is  connected 
with  the  voltmeter.  Increasing  current  due  to  increasing 
load  affects  the  voltmeter  in  such  a  manner  as  to  require 
increased  current  through  the  main  voltmeter  solenoid  in 
order  to  give  the  same  resultant  pull  on  the  core,  i.e.,  the 
same  reading. 

The  connections  of  a  compensator  invented  by  Mr. 
Ralph  Mershon  with  accompanying  transformer  and  volt- 
meter are  shown  in  Fig.  116.  The  instrument  is  also 
made  to  compensate  for  inductive  as  well  as  ohmic  drop. 

Rectifiers.  —  A  rectifier  is  a  device  for  changing  an 
alternating  current  into  a  direct  current,  and  is  intended 
mainly  for  the  operation  of  series  arc  lamps.  The  ap- 
paratus usually  consists  of  a  constant-current  transformer, 
giving  constant  alternating  current  at  all  loads,  and  a 


1/6   POLYPHASE  APPARATUS  AND  SYSTEMS. 

rectifying  device  which  runs  in  synchronism  with  the 
alternating,  and  converts  the  constant  alternating  current 
into  a  direct  current,  but  more  or  less  pulsating. 

At  light  loads  the  rectifier  has  an  idle  current  of  nearly 
100  per  cent  of  full-load  current,  and  at  full  load  a  low 
power-factor.  P"or  polyphase  circuits,  therefore,  to  avoid 


Fig.  116. 

unbalancing  of  the  phases,  the  rectifier  should  preferably 
be  of  polyphase  design. 

Frequency  Changer In  alternating-current  plants,  em- 
ploying a  low  frequency,  there  is  sometimes  a  need  for  a 
limited  amount  of  current  of  a  higher  frequency.  For 


STATION   EQUIPMENT  AND  APPARATUS.       177 

instance,  in  25  and  40  cycle  installations,  incandescent  and 
arc  lighting  may  be  required.  To  meet  such  cases  a  fre- 
quency of  60  cycles,  or  any  other  number  of  cycles  suit- 
able for  lighting,  may  be  obtained  economically  and  cheaply 
by  means  of  a  frequency  changer.  This  is  essentially  an 
induction  motor,  the  armature  of  which  is  rotated  by  a 
synchronous  motor  in  a  direction  usually  opposite  to  its 
natural  rotation.  The  lower  frequency  current  is  fed  to 
the  primary  or  field,  and  the  current  at  the  higher  fre- 
quency is  taken  out  of  the  secondary  or  armature  by  means 
of  collector  rings.  The  frequency  and  voltage  of  the  out- 
put will  depend  on  the  speed  of  the  secondary,  and  will  be 
the  algebraic  sum  of  the  current  pulsations  in  both  mem- 
bers. If  the  secondary  is  run  at  rated  speed,  but  in  oppo- 
sition to  its  natural  rotation,  the  frequency  will  be  twice 
that  of  the  normal  current,  or  if  run  at  one-half  speed  in 
its  natural  direction,  the  frequency  will  be  one-half  the 
normal.  To  change  a  frequency  of  40  cycles  to  60  cycles, 
the  secondary  would  be  run  at  one-half  speed  in  an  oppo- 
site direction,  while  to  obtain  60  cycles  from  a  2 5 -cycle 
current,  the  secondary  would  run  nearly  two  and  one-half 
times  the  rated  speed  in  an  opposite  direction. 

The  capacity  of  the  driving-motor  end  of  the  frequency 
changer  bears  the  same  proportion  to  the  total  output  that 
the  increase  in  frequency  bears  to  the  final  frequency. 
The  secondary  of  the  frequency  changer  proper  must  equal 
the  output.  The  capacity  of  the  primary  has  the  same 
proportion  to  the  total  output  that  the  initial  frequency  has 
to  the  final  frequency.  As  an  illustration,  —  a  100  K.W. 
frequency  changer,  primary  40  cycles,  secondary  60  cycles, 
would  be  composed  as  follows  :  A  40  cycle  synchronous 
motor,  capacity  33  K.W.,  speed  600  R.P.M.,  direct-con- 


1/8       POLYPHASE   APPARATUS   AND    SYSTEMS. 

nected  to  the  secondary,  capacity  100  K.W.  Primary  capa- 
city would  be  66  K.W.  The  primary  would  be  four  polar. 
The  natural  speed  of  the  secondary  would  be  i, 200  R. P.M. 
By  driving  it  at  a  speed  of  600  R.P.M.  in  the  opposite 
direction  to  its  natural  rotation,  the  number  of  reversals 
will  be  that  due  to  an  equivalent  speed  of  1,800  R.P.]\I., 
or  60  cycles.  For  the  sake  of  illustration,  the  capacities 


Fig-.  117. 

as  given  above  are  on  the  assumption  of  a  100  per  cent 
efficiency,  which,  of  course,  is  an  impossibility.  Fig.  117 
depicts  the  general  form  of  this  apparatus. 

Motor  Generators.  —  A  motor  generator  for  alternating 
current  work  consists  of  an  induction  or  synchronous 
motor  driving  a  generator.  The  motor  is  usually  mounted 
on  the  same  base  and  direct  connected  to  the  generator. 

It  may  perform  the  functions  of  a  frequency  changer,  in 


STATION    EQUIPMENT   AND   APPARATUS.       179 

which  case  the  generator,  of  course,  is  of  the  alternating 
current  type,  or  it  may  be  used  in  place  of  a  rotary  con- 
verter, the  generator  then  delivering  a  direct  current. 
Although  more  expensive  and  less  efficient  than  a  rotary 
converter,  the  motor  generator  has  the  advantage  of  not 
always  requiring  step-down  transformers.  It  is  self-regu- 
lating, and  is  not  materially  affected  by  potential  fluctua- 
tions of  the  transmission  lines. 

The  induction  motor  set  possesses  one  great  advantage 
over  the  synchronous  motor  set,  in  that  it  is  entirely  free 
from  any  tendency  to  hunt.  This  condition  of  instability 
is.  most  apparent  in  the  case  of  motor  generators  operating 
at  the  ends  of  long-distance  transmission  lines.  The  cures 
for  this  trouble  have  already  been  described. 


l8o      POLYPHASE   APPARATUS   AND    SYSTEMS. 


CHAPTER  IX. 
TWO-PHASE  SYSTEM. 

Polyphase  Systems  and  Combinations. —  Any  arrange- 
ment of  conductors,  carrying  two  or  more  single-phase 
alternating  currents,  definitely  related  to  one  another  in 
point  of  time,  constitutes  a  polyphase  system.  The  sys- 
tems commonly  employed  for  the  generation  and  distribu- 
tion of  power  by  polyphase  currents  are  the  two-phase, 
three-phase,  and  a  third,  a  combination  of  a  single-phase 
and  polyphase  conductor  arrangement,  called  the  mono- 
cyclic  system. 

Polyphase  currents  are  usually  produced  by  alternators, 
the  armatures  of  which  are  so  wound  that  the  electro- 
motive forces  at  the  terminals  correspond  to  the  number 
of  phases,  and  arrive  at  a  maximum  in  a  fixed  and  definite 
relation  to  one  another. 

In  the  two-phase  system  the  two  electro-motive  forces 
and  currents  are  90°,  or  one-fourth  of  a  cycle,  apart.  The 
relations  of  the  curves  to  each  other,  and  their  instantaneous 
values,  can  be  seen  from  the  development  of  the  diagram  of 
single  harmonic  motion  (Fig.  118).  The  maximum  of  one 
wave  occurs  when  the  value  of  the  other  is  zero.  If  the 
pressure  in  any  one  of  the  coils  Oa  or  Ob  is  I,  the  pressure 
between  the  ends  ab  is  V~2  =  1414. 

The  windings  of  a  polyphase  machine  may  be  combined 
to.  a  number  of  ways,  each  affecting  the  relation  of  the 


TWO-PHASE   SYSTEM. 


181 


electro-motive  forces  of  the  outside  conductors,  as  shown 
in  Figs.  119  to  123.  These  diagrammatically  represent 
the  coils  of  a  two-phase  machine,  in  which  the  electro- 


Fig.  118. 

motive  forces  may  be  considered  as  being  either  generated 
or  absorbed.  In  Fig.  119  all  the  coils  are  in  series,  form- 
ing a  continuous  winding,  tapped  at  four  points.  This 
arrangement  is  known  as  an  interlinked  winding.  Leads 
I  and  2  constitute  the  circuit  of  one  phase,  and  3  and  4 
that  of  the  second  phase.  The  E.M.F .  between  the  wires 
of  different  phases  in  1.4 -=-2  times  that  between  leads  of  the 
same  phase.  In  Fig. 
1 20  the  windings  of 
each  phase  are  sepa 
rate.  This  arrange- 
ment can  be  made  in- 
terlinked by  joining  the 
two  circuits  where  they 
cross,  thus  forming  a 
common  centre,  as 


shown     in    Fig.     121. 


Fig.  119. 


The  relation  of  E.M.F. 
is  the  same  as  in  Fig.  120.  The  grouping  of  coils,  shown 
in  Fig.  1 20,  may  also  be  made  interlinked  by  joining  leads 
4  and  2  (Fig.  122),  which  become  a  common  return  for  I 


l$2   POLYPHASE  APPARATUS  AND  SYSTEMS. 

and  3.  The  E.M.F.  between  the  two  outgoing  wires  is  1.4 
times  that  between  each  outgoing  wire  and  the  common 
return. 

The  windings  of  interlinked  systems  are  classed  accord- 
ing to  their  connections  as  "Ring,"  or  "  Star."  Figs.  119 
and  1 2 1  respectively,  show  the  ring  and  star  connections  of 
the  two-phase  system. 

In  the  three-phase  system,  the  ring  and  star  connections 


Fig.  120. 


Fig.  121, 


are  usually  designated  as  Y  and  A  (Delta),  from  their 
resemblance  to  these  symbols. 

The  winding  connections  of  most  commercial  two-phase 
machines  are  interlinked.  Fig.  123  shows  the  connections 
of  a  Westinghouse  two-phase  2,000  volt  generator.  Con- 
nections are  made  to  the  winding  at  four  points. 

The  current  in  the  circuit  1-3  is  90°  apart  from,  or  in 


TWO-PHASE   SYSTEM. 


183 


rpr 

T 

0 

o 

f 

1                -     I 

o 

Qt 
1 

t 
O 

o 

! 

o 

<a 

o 

«y< 

| 

1 

i                               1 

1 84   POLYPHASE  APPARATUS  AND  SYSTEMS. 

quadrature  with,  the  current  in  the  circuit  2-4.  The  E.M.F. 
existing  between  any  two  adjacent  terminals  is  1,400  volts. 
If  the  E.M.F.  is  raised  or  lowered,  the  same  proportions 
hold;  and  for  a  1,000  volt  machine,  the  electro-motive 
forces  are  respectively  1,000  and  700  volts. 

No  matter  what  the  arrangement  of  the  winding  may 
be  in  a  polyphase  machine,  whether  the  coils  are  inter- 
linked, or  separately  grouped,  ring  or  star  connected,  the 
principles  of  action  are  the  same,  and  the  characteristic 
polyphase  results  are  equally  present. 

Polyphase  systems  have  two  desirable  features  :  First, 
the  supply  of  power  is  continuous  and  uniform,  thus 
increasing  the  capacity  of  apparatus,  and  in  some  systems, 


1000 


^         1QO 

1 

3 

<,        I 

<T          100 

>     i 

2 
4 

Fig.  124. 

that  of  transmission  conductors  ;  and,  second,  the  use  of 
revolving  types  of  induction  apparatus  is  permitted,  which 
do  not  require  any  form  of  moving  contacts. 

Transformer  Connections.  —  A  number  of  combinations 
of  two-phase  circuits  can  be  made  by  suitably  arranging 
transformers  with  due  regard  to  the  generator  windings. 
Fig.  124  shows  the  connections  commonly  used  for  light- 
ing and  transmission  of  power.  The  arrangement  consists 
of  two  single-phase  transformers,  the  phases  being  sepa- 
rated in  both  primary  and  secondary.  Two  of  the  sec- 
ondary leads  are  sometimes  joined  (Fig.  125),  making  a 


TWO-PHASE    SYSTEM. 


I85 


common  return  for  the  other  wires.  The  two  circuits  being 
90°  apart,  the  voltage  between  i  and  4  is  ^J~2  times  that 
between  the  outside  wires  and  the  common  return.  This 
arrangement  is  best  adapted  for  supplying  current  of  mini- 
mum potential  to  apparatus  in  the  vicinity  of  the  trans- 
formers. It  is  more  frequently  used  in  connection  with 
motors  operating  from  the  secondaries  of  the  transformers. 


f  .  ,; 

1  1 

° 

af 

2    1 

1  o'oo        B 


6    160 


Fig.  125. 


1 

<> 

A 

10 

00 

1 

3 

; 

2 

, 

•-, 

14 

00 

,/" 

p 

10 

00 

< 

4      N 

^ 

f 

<f 

4     I 


> 

1 

>        Cl       1 

DO 

^—  I 

3 

I 

2 

>      b     1 

DO 

| 

^ 

4 

Fig.  126. 

Fig.  126  shows  another  arrangement  of  transformers  where 
the  common  return  is  used  on  both  primary  and  secondary. 
As  will  be  explained  farther  on,  this  connection  is  permis- 
sible only  when  the  power  of  the  two  circuits  is  consumed 
by  one  unit,  or  when  both  sides  of  the  system  are  bal- 
anced. 

Two-Phase  to  Three-Phase.  —  It  is  possible,  by  a  combi- 
nation of  two  transformers,  to  change  one  polyphase  sys- 
tem into  any  other  polyphase  system.  The  transformation 


1 86   POLYPHASE  APPARATUS  AND  SYSTEMS. 


from  two-phase  to  three-phase,  or  vice  versa,  is  effected 
by  proportioning  the  windings,  as  shown  in  Fig.  127.  One 
transformer  is  wound  with  a  ratio  of  transformation  of 
1,000  to  100;  the  other  with  a  ratio  of  1,000  to  86.7.  The 
secondary  of  this  transformer  is  connected  to  the  middle 
of  the  secondary  winding  of  the  first.  In  Fig.  128,  AB 

represents  the 


10 

00                                ?     'S  100 

from  A  to  B  in 
D     one      transfor- 

|                   1C 

o 
K 

mer.    At  right 
angles   to    AB 
^    the  line  CO  re- 

10 

OO                                >      <  86.7 

Fig.  127.                                   presents,  in  di- 
rection     and 

quantity,  the  pressure  O  to  C  of  the  second  transformer. 
From  the  properties  of  the  triangle  it  follows  that,  at  the 
terminals  A,  B,  C,  three  equal  pressures  will  exist,  each 
differing  from  the  others  by  60  ,  and  giving  rise  to  a  three- 
phase  current. 

For  this  transformation  on  a  small  scale,  it  is  customary 
to   use   standard   transformers,   the 
main   transformer  having  a  ratio  of 
10  to  i,  and  the  teaser  a  ratio   of 
9  to  i. 

The  current  in  the  winding  OC, 
being  a  resultant  of  the  other  two 
phases,  is  greater  than  if  the  change 
to  three-phase  were  not  made ;  and, 
consequently,  for  the  same  heating, 
necessitates  more  transformer  capacity.  Only  one  trans- 
former, the  teaser,  need  be  of  greater  output.  The  increase 


TWO-PHASE    SYSTEM. 


I87 


is  in  the  secondary,  being  1 5  per  cent,  or  about  4  per  cent 
of  the  total  transformer  capacity.  If  the  transformers  are 
interchangeable,  the  excess  capacity  required  in  the  two 
transformers  is  over  12  per  cent.  The  secondary  of  each 
interchangeable  transformer  has  two  taps,  giving  50  per 
cent  and  86.7  per  cent  of  the  full  voltage,  so  that  either 
transformer  can  serve  as  the  teaser,  or  supplementary  one, 
by  using  the  proper  terminals. 

In  the  long-distance  transmission  of  power  the  genera- 
tors are  sometimes  wound  two-phase,  and  the  secondary 


Fig.  129. 

distribution  at  the  receiving  end  is  likewise  by  the  two- 
phase  system,  while  on  account  of  the  saving  in  copper  the 
transmission  is  by  the  three-phase  system.  Such  is  the  ar- 
rangement of  the  apparatus  at  the  generating  end  of  the 
Niagara-Buffalo  plant.  The  distribution  in  Buffalo,  how- 
ever, is  mainly  by  the  three-phase  system.  Fig.  129  shows 
the  transformer  connections  for  changing  two-phase  to 
three-phase  and  back  again. 

Two-Phase  Four-Wire  System This    system    consists 

of  two  separate  circuits,  derived  from  two  independent 
armature  windings  in  quadrature  with  each  other,  or  from 
a  continuous  armature  winding  tapped  at  four  equidistant 


188   POLYPHASE  APPARATUS  AND  SYSTEMS. 

points.  The  practical  application  of  this  system  is  illus- 
trated in  Fig.  130.  Each  of  the  two  generators  A  and  B 
delivers  two-phase  currents  of  low  potential  to  the  step-up 
transformers  RT,  RT',  RT",  RT'",  through  the  switch- 
board D.  The  transmission  lines  L,  L',  L",  L'",  receive  and 
transmit  current,  at  a  high  pressure,  to  a  substation  con- 
veniently located  with  reference  to  the  districts  where  lights 
and  motors  are  to  be  supplied.  The  high-potential  current 
is  here  reduced  by  the  transformers  L  T,  L  T',  L  T",  L  7'"',  to 
a  commercial  pressure  suitable  for  local  distribution,  through 
the  switchboard  F.  Beginning  at  the  right  of  the  figure, 
the  first  four-wire  system  is  used  to  supply  alternating  cur- 
rent to  the  rotary  converter,  which,  in  turn,  delivers  direct 
current  at  500  volts  to  a  trolley  line  operating  the  street- 
car systems  K.  The  second  circuit  supplies  the  motors  M, 
M',  M",  M'",  either  of  the  synchronous  or  induction  type. 
The  next  four-wire  system  is  divided  into  two  distinct 
circuits,  supplying  current  to  incandescent  lamps  through 
the  transformers  b,  b',  B",  B'" .  The  next  circuit  supplies 
current  for  arc  lighting  through  a  rotary  converter.  An- 
other rotary  converter  is  operated  from  the  last  circuit, 
and  delivers  low-voltage  current  for  electrolytic  purposes. 
The  rotary  converters  in  practice  are  supplied  with  trans- 
formers, not  shown  in  the  diagram,  which  deliver,  at  the 
rotary  terminals,  an  alternating  current  of  the  proper  vol- 


tage. 


The  two  single  circuits  must  be  balanced  as  nearly  as 
possible,  and  for  this  purpose  the  four  wires  must  be  car- 
ried through  the  same  district  to  be  supplied  with  power 
or  light.  In  order  to  obtain  economy  in  copper  in  a  sec- 
ondary system  of  distribution,  it  is  desirable  to  use  three- 
wire  mains.  In  the  two-phase  four-wire  system,  where 


TWO-PHASE    SYSTEM 


189 


190 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


motors  are  to  be  supplied,  the  two  independent  three- 
wire  circuits  must  be  brought  together,  making  six  wires 
in  all. 

The  measurement  of  power  by  this  system  is  obtained 
by  the  use  of  a  wattmeter  inserted  in  each  circuit,  as  in  a 
single-phase  system.  The  sum  of  the  two  readings  gives 


AAW 


w/vwv 

JWWAJWAM 


M/w)         \MMV\Ww 


WvVwWv 
/WWJNAM 


Fig.  131. 

the  total  power  supplied.     In  a  balanced  system,  twice  the 
reading  of  one  wattmeter  will  give  the  power. 

Two-Phase  Three-Wire  System. — By  joining  any  two 
conductors  in  the  four-wire  system,  a  common  return  is 
made  for  the  two  circuits.  This  arrangement  of  circuits 
is  called  the  two-phase  three-wire  system.  As  previously 
shown,  the  pressure  between  the  common  conductor  and 


TWO-PHASE    SYSTEM,  IQI 

the  others  is  42  per  cent  higher  than  that  which  existed 
before.  With  a  given  load  and  insulation  strain,  the  com- 
mon conductor  must  be  made  larger  in  proportion,  in  order 
to  keep  the  loss  the  same. 

The  general  application  of  this  system  is  shown  in  Fig. 
131.  Two  terminals  of  the  generator  coils  are  united;  and 
the  three  leads,  forming  an  interconnected  two-phase  sys- 
tem, are  run  to  wherever  motors  and  lights  are  to  be  sup- 
plied. When  motors  are  used,  connection  is  made  directly 
with  the  main  leads,  or,  if  the  motors  are  wound  for  low 
voltage,  connection  is  made  through  two  transformers. 
The  motors,  which  are  of  the  ordinary  two-phase  type,  may 
have  their  terminals  connected  either  on  the  three-wire  or 
four-wire  system. 

Where  lights  are  supplied,  the  transformers  may  be  con- 
nected singly  to  only  one  circuit,  or  in  pairs  on  two  cir- 
cuits, with  a  common  return.  In  practice,  it  is  essential 
that  both  phases  be  equally  loaded. 

In  this  arrangement  of  conductors  there  is  an  unbalan- 
cing of  both  sides  of  the  system  on  an  inductive  load,  which 
exists,  even  though  the  energy  load  is  equally  divided. 
This  unbalancing  is  due  to  the  fact  that  the  E.M.F.  of  self- 
induction  in  one  side  of  the  system  is  in  phase  with  the 
effective  E.M.F.  in  the  other  side,  thus  distorting  the  uni- 
form current-distribution  in  both  circuits. 

The  distribution  of  currents  and  electromotive  forces  in 
the  three  conductors  in  the  single-phase  three-wire,  the 
three-phase  and  the  two-phase  three-wire  system,  is  shown 
in  the  following  table.  The  figures  are  the  results  of  ex- 
periments to  determine  the  self-induction  of  underground 
tubes. 


192       POLYPHASE  APPARATUS   AND    SYSTEMS. 


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TWO-PHASE   SYSTEM.  193 

The  single-phase  and  the  three-phase  systems  give  equal 
drops,  but  the  induction  unbalancing  of  the  two-phase 
three-wire  system  is  beyond  the  range  of  practical  opera- 
tion. These  results  were  obtained  with  low-tension  sys- 
tems and  moderate  drops.  The  unbalancing  effect  is  much 
greater  with  higher  voltage  and  drops.  The  four-wire  two- 
phase  system  would,  of  course,  show  no  such  unbalancing. 


194       POLYPHASE   APPARATUS   AND    SYSTEMS. 


CHAPTER    X. 
THREE-PHASE   SYSTEM. 

Curves  of  E.M.F.  —  The  E.M.F.  impulses  in  a  three- 
phase  system  follow  one  another  at  intervals  of  60°,  The 
instantaneous  values  and  the  relation  of  the  phases,  devel- 
oped from  the  diagram  of  simple  harmonic  motion,  are  shown 
in  Fig.  132.  The  curves  Oa,  Ob,  Oc,  represent  the  electro- 
motive forces  produced  by  three  sets  of  generator  coils.  If 
the  distance  from  O  to  a,  b,  and  c,  be  taken  equal  to  i,  it 


60 


Fig.  132. 

follows  from  the  diagram  that  the  lines  joining  -a,  b,  and  c 
are  equal  to  V  3  =  1-732.  That  is,  the  pressure  between 
the  ends  of  any  two  of  the  generator  coils  in  a  three-phase 
system  is  1.732  times  that  between  the  common  juncture 
O  and  the  terminals  of  the  coils. 

It  will  be  seen  from  the  diagram  that  each  one  of  the 
coils  successively  serves  as  a  return  for  the  other  two,  and 
that  the  algebraic  sum  of  the  currents  in  the  system  is 


THREE-PHASE    SYSTEM. 


195 


zero.  The  three-phase  system  may  be  resolved  into  three 
single  circuits,  with  a  common  or  grounded  return.  The 
sum  of  'the  currents  being  zero,  no  current  will  flow  in  the 
return  conductor,  and  it  may  be  dispensed  with.  The 
system  then  becomes  the  ordinary  F-connected  arrange- 
ment. 

Transformer  Combinations.  — The  ring  and  the  star  con- 
nections of  three-phase  windings,  —  whether  of  armature  in 


PRIMARY 


SECONDARY 


Fig.  133. 


which  the  electromotive  forces  are  induced,  or  transformer, 
or  motor  in  which  the  electromotive  forces  are  absorbed, 
—  are  designated  by  the  symbols  A  and  Y  respectively. 
Figs.  133  to  137  illustrate  the  various  three-phase  combi- 
nations of  single-phase  transformers  in  practical  operation. 
Fig.  133  shows  A  connection  of  both  primary  and  secon- 
dary terminals  of  transformers,  having  a  ratio  of  10  to  I. 
Fig.  134  shows  three  transformers,  Y  connected  in  both 
windings.  The  ratio  of  pressures  between  any  two  corre- 
sponding terminals  in  primary  and  secondary  are  the  same 
as  in  the  A  arrangement.  The  individual  transformers 


196       POLYPHASE   APPARATUS   AND    SYSTEMS. 

thus  connected  have  fewer  turns  for  the  same  voltage  than 
when  A  connected,  and  thus  this  arrangement  is  suitable  for 
very  high  line-pressures.  Fig.  135  shows  a  combination  of 
A  and  V  connection,  the  primaries  of  the  transformers  being 


PRIMARY 


SECONDARY 


Fig.  134. 


PRIMARY 


SECONDARY 


Fig.  135. 


connected  A,  while  the  secondaries  are  connected  Y.  A 
fourth  wire  may  be  led  from  the  common  centre  of  the 
three  secondaries.  The  pressure  between  this  neutral  and 

any  one  of  the  outside  wires  is  — —  of  the  pressure  between 


THREE-PHASE    SYSTEM. 


197 


the  outside  wires.  This  arrangement  is  known  as  the 
"three-phase  four-wire  system,"  and  is  especially  conven- 
ient and  economical  in  secondary  distributing  systems.  In 


PRIMARY 


SECONDARY 


Fig.  136. 


PRIMARY 


SECONDARY 


Fig.  137. 


Fig.  136,  the  primaries  are  connected  F,  the  secondaries 
A.  The  A  connection  is  sometimes  made  up  of  two  trans- 
formers (Fig,  137),  instead  of  three.  The  pressures  be- 


198   POLYPHASE  APPARATUS  AND  SYSTEMS. 

tween  all  three  terminals  are  equal,  that  from  the  open 
side  of  the  triangle  being  the  resultant  of  the  E.M.F.  in 
the  existing  windings.  This  arrangement  is  frequently 
used  with  motors,  its  chief  advantages  being  its  simplicity, 
and  permitting  the  use  of  available  transformers,  when  the 
motor  cannot  be  fitted  with  three  transformers  of  exactly 
the  capacity  wanted.  Its  disadvantage  is  that  the  motor 
will  stop  in  case  of  accident  to  one  transformer.  The  com- 
bination of  three  transformers,  arranged  in  A,  is  most  con- 
venient and  desirable,  for  the  reason  that  an  accident  to 
one  does  not  interrupt  the  service  ;  the  only  requirement 
being,  that  the  load  be  reduced  one-third,  to  prevent  heat- 
ing of  the  transformers.  Another  disadvantage  of  the  re- 
sultant A,  arrangement  is  the  increased  transformer  capacity 
required,  as,  for  the  same  total  energy,  the  flow  of  current 
is  increased  through  the  two  existing  secondaries.  This 
disadvantage  is  not  so  noticeable  in  small  transformers,  but 
must  be  allowed  for  when  working  with  large  transformers. 
Six-Phase  Transformer  Connections.  —  A  six-phase  ar- 
rangement is  now  used  to  a  great  degree  for  the  supply 
of  rotary  converters.  It  is  derived  either  from  a  three- 
phase  transformer  or  a  combination  of  three  single 
transformers  connected  in  three-phase.  In  such  an  in- 
stallation for  a  rotary  converter  there  is  required  six 
leads  from  the  transformers  connected  to  six  collector 
rings  tapped  to  six  points  in  the  armature  winding  for 
each  pair  of  poles.  Fig.  138  shows  one  arrangement, 
which  is  the  six-phase  equivalent  of  the  two-phase  rotary 
converter.  One  secondary  for  each  single-phase  trans- 
former is  needed.  Fig.  1 39  shows  the  six-phase  equivalent 
of  the  three-phase  converter.  This  arrangement  requires 
two  electrically  separate  secondaries,  which  are  here  con- 


THREE-PHASE    SYSTEM. 


199 


nected  in  delta  fashion.  An  equivalent  combination  is 
that  in  which  the  separate  secondaries  are  connected 
in  K 

The  terminals  of  each  transformer  in  Fig.  138  are  con- 
nected to  the  converter  windings  at  points  180°  apart. 
Hence  it  is  known  as  the  diametrical  six-phase  connection. 
The  ratio  between  the  alternating  and  direct  current 
potentials  of  a  converter  so  connected  up  is  the  same  as 


WWWVWWv       VWM/WWVW      VWWVWAW 

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J 


Fig-.  138. 


Pig-.  139. 


that  of  a  straight  two-phase  converter.  The  diametrical 
is  the  simplest  six-phase  arrangement. 

In  the  delta  six-phase  connection  (Fig.  139)  the 
E.M.F.'s,  1 80°  out  of  phase,  are  derived  from  each  single- 
phase  transformer  by  reversing  the  connections  of  the 
two  separate  secondaries.  The  six  transformer  terminals 
are  progressively  connected  in  such  a  way  that  the  re- 
sultant effect  is  a  double  delta,  which  yields  the  same 
ratio  of  conversion  as  a  plain  three-phase  converter  con- 
nection. 

Motor  Connections Motors  are  connected  to  the  sec- 
ondaries of  three  transformers  in  a  three-phase  system,  as 
shown  in  Fig.  140. 


200      POLYPHASE   APPARATUS   AND    SYSTEMS. 

The  primaries,  /-i,  /-2,  /-3,  of  three  transformers 
are  connected  between  the  three  lines  A,  B,  C,  leading 
from  the  generator,  and  three  secondaries,  5-1,  5-2, 
5-3,  are  connected  in  delta  to  the  three  lines,  a,  b,  c, 
leading  to  the  motor.  A  recording  wattmeter  of  the 
three-phase  type,  for  measuring  the  power  consumed  by 
the  motor,  is  shown  connected  in  the  system  with  the 


P1 

r~ 

A 

B' 
Generator 

n  
p2 
i  — 

C 

*n  

pa 

Generator 


Motor 


Fig.  141. 

field  spools  at  /,  the  armature  circuit  ar  and  its  resistances 
r,  between  the  three  secondary  lines. 

Induction  motors  may  be  supplied  from  a  three-phase 
generator  by  means  of  two  reducing  transformers  in  the 
manner  shown  in  Fig.  141.  This  arrangement  is  identical 
with  that  in  Fig.  140,  except  that  one  of  the  transformers, 
P-3,  5-3,  is  left  out,  and  the  two  other  transformers 
are  made  correspondingly  larger.  The  recording  watt- 


THREE-PHASE    SYSTEM. 


201 


meter  is  connected  in  the  secondary  circuit  in  the  same 
way  as  in  the  use  of  three  transformers. 

The  connections  of  three  transformers  for  a  low-tension 
distribution  system,  by  the  three-phase  four-wire  system, 
are  shown  in  Fig.  142.  The  three  transformers  have 
their  primaries,  P—i,  P-2,  P~$,  joined  in  delta  connection, 
and  their  secondaries,  S-i,  S-2,  5- 3,  in  Y  connection. 
Lines  a,  b,  e,  are  the  three  main  three-phase  lines,  and  d  is 


Generator 


>            a 
g:  S1 

~—  .      . 

T 
20})  V 

>      "    b 

^  S2 

> 

—  •  ! 

{      ^ 

t     20t>  V 

H 

>      '      c 
g  S3 

t        \    11?V 
11I5V 

T  i    1 

d 

Fig.  142. 

the  common  neutral.  The  difference  of  potential  between 
a  and  b,  b  and  c,  and  a  and  c  is  200  volts,  while  that  be- 
tween them  and  d  is  115  volts.  200  volt  motors  are  joined 
to  a,  b,  and  c,  while  1 1 5  volt  lamps  are  connected  between 
a  and  d,  b  and  d,  or  c  and  d.  Line  d,  like  the  neutral 
wire  in  the  Edison  three-wire  system,  only  carries  current 
when  the  lamp  load  is  unbalanced. 

Measurement  of  Power.  —  In  a  F-connected  generator  the 

j? 
E.M.F.,  induced  in  each  phase,  is-—,  and  the  energy  in 

E  ^3 

that  phase  is  /  x  —=L,  E  being  the  E.M.F.  at  the  generator 

Vs 
terminals.     In  a  A  connected  generator  the  current  in  each 

phase  winding  is — -,  /being  the  line  current,  and  the  energy 


202   POLYPHASE  APPARATUS  AND  SYSTEMS. 

is  E  x  — -.     The  total  energy  for  the  three  phases,  in  the 

^3 

cases  both  of  a  Y  and  a  A  connected  generator,  is  =  V3  x  E 
X  /.  This  formula  is  correct  when  the  generator  output 
is  of  a  non-inductive  character.  If  a  phase  displacement 
exists,  the  expression  becomes  \/3  x  E  x  /  X  Cos  <f>.  These 
formulas  apply  equally  well  for  determining  the  power 
in  a  three-phase  circuit,  irrespective  of  the  method  of 
connections  of  the  supplying  source  or  of  the  consuming 
devices. 

As  an  illustration,  —  the  power  in  a  non-inductive  three- 
phase  circuit,  in  each  branch  of  which  100  amperes  are 
flowing,  the  voltage  between  lines  being  2,500,  is  found  as 
follows:  the  energy  in  each  phase  is  =  100  amperes  x  2,500 
volts  x  V3  ==  145  K.W.,  and,  for  the  three  circuits,  is 
therefore  435  K.W.  If  the  circuit  had  a  power  factor 
of  80  per  cent,  the  energy  would  then  be  435  x  .80 
=  348  K.W. 

The  power  supplied  by  three-phase  circuits  can  be  meas- 
ured by  the  use  of  three,  two,  or  one  wattmeter.  Three 
wattmeters  will  give  the  power  of  a  circuit  irrespective  of 
the  condition  of  balancing  or  lag.  The  sum  of  the  read- 
ings of  the  three  instruments  is  the  total  power.  Each 
wattmeter  must  be  connected  to  the  common  centre  or 
neutral  of  the  system.  If  the  apparatus  is  connected 
delta,  it  is  necessary  to  make  an  artificial  neutral  with 
resistances.  Two  wattmeters  can  be  connected  so  that,  as 
long  as  the  power  factor  is  greater  than  50  per  cent,  the 
sum  of  the  two  readings  equals  the  total  power.  The  dif- 
ference of  the  two  readings  will  give  the  power  when  the 
power  factor  is  less  than  50.  As  it  is  not  possible  to  tell 
when  the  power  factor  falls  below  this  point,  without 


THREE-PHASE   SYSTEM.  203 

reversing  the  connections,  this  method  is  inconvenient 
and  undesirable. 

When  three-phase  circuits  are  in  balance  in  respect  to 
load  and  power  factor  the  power  may  be  measured  by  one 
wattmeter.  Three  times  the  readings  of  the  single  watt- 
meter will  give  the  total  power  in  the  circuits.  Figs.  143 
and  144  show  the  connections  of  three-phase  recording 
wattmeters  for  low  and  for  high  voltage  circuits.  The 
wattmeter  is  provided  with  resistances,  r,  r  and  r1,  for 
creating  an  artificial  neutral.  The  armature  windings  are 
in  series  with  r\  so  that  r'  +  a  =  r.  The  wattmeter,  dia- 
grammatically  illustrated  in  Fig.  -143,  is  adapted  for 
circuits  of  550  volts  and  less.  Fig.  144  shows  the  con- 
nection of  a  wattmeter  for  circuits  of  from  1,000  to  3,000 
volts.  Station  transformers  t  and  /  are  required  to  reduce 
the  pressure  for  the  armature  windings. 

The  connections  for  an  indicating  wattmeter  are  the 
same  as  those  for  a  recording  wattmeter.  The  main  cur- 
rent is  taken  by  the  stationary  or  low-resistance  coil,  while 
the  pressure  coil  is  of  high  resistance,  and  connected  to 
the  artificial  neutral. 

The  power  in  unbalanced  three-phase  circuits  is  meas- 
ured by  a  special  form  of  wattmeter  of  the  induction 
type.  This  wattmeter  is  also  largely  used  in  balanced 
circuits. 

Three-Phase  Circuits The  general  arrangement  of 

circuits  for  a  local  distribution  of  light  and  power  is 
shown  in  Fig.  145.  The  generators  are  wound  for  2,000 
volts,  feeding  direct  into  the  mains.  Step-down  trans- 
formers reduce  the  power  to  100  volts  for  lights  and  200 
volts  for  motors.  In  one  arrangement  alternating  enclosed 
arc  lights  are  shown,  operated  from  a  transformer.  A  200 


204      ^OLYPHASE   APPARATUS   AND    SYSTEMS. 

r 


To  Generator 


To  Generator 


110-220-550  Volts 
Fig.  143. 


To  Lint. 


To  Line 


1 150-2300  Volts 
Fig.  144. 


THREE-PHASE    SYSTEM. 


205 


volt  motor  is  supplied  by  three  transformers,  constituting 
a  system  of  secondary  mains.  In  the  second  arrangement 
a  motor  running  from  two  transformers,  and  a  general  dis- 
tributing system,  are  shown.  The  general  practice  is  to 
wind  the  generators  for  1,040  or  2,080  volts,  no  load,  and 
use  transformers  reducing  to  1 1 5  volts  for  light  and  small 


Fig.  145. 

motors.     Where  secondary  mains  are  employed  the  motor 
pressure  is  200,  220,  440,  or  550  volts. 

Where  lights  and  motors  are  located  a  considerable  dis- 
tance from  the  generators,  the  cost  of  copper  is  reduced  by 
employing  transformers  to  raise  the  current  pressure.  An 
arrangement  of  three-phase  circuits  for  transmitting  power 
over  long  distances  is  shown  in  Fig.  146.  The  generator, 
direct-connected  to  the  source  of  power,  a  water-wheel,  is 
shown  at  A.  B  is  a  bank  of  step-up  transformers,  raising 


206       POLYPHASE   APPARATUS   AND    SYSTEMS. 

the  voltage  to,  say,   20,000.      As  this    voltage  is  higher 
than  can  be  used  with  any  apparatus  for  direct  utilization 


of  the  current,  step-down  transformers,  C{,  C.,,  and  C3,  are 
required. 

The  main  substation  contains  the  transformers,  Cl  and 
C4.     This  is  a  true   central  or  distributing  station.     From 


THREE-PHASE    SYSTEM.  2O/ 

this  point  the  distributing  feeders  are  taken  out  at,  say, 
2,080  volts,  for  the  commercial  primary  circuits  and  through 
the  bank  C2  at  115  volts,  to  feed  a  low-tension  network. 
Through  the  transformer  Clt  a  current  of  2,080  volts  is 
fed  direct  into  a  synchronous  motor,  and  into  transformers 
reducing  to  115  volts  for  supplying  motors  and  lights. 
The  substation  transformers  C9  furnish  current  for  a 
general  lighting  and  motor  service  at  /,  /,  and  H.  The 
voltage  for  this  distributing  system  is  controlled  by  the 
regulators  G. 

At  Cz  another  bank  of  step-down  transformers  is 
located.  An  alternating  current  of  suitable  voltage  is  de- 
livered to  the  rotary  converter  D,  which  supplies  contin- 
uous current  to  the  electrolytic  vats  or  storage  battery  E. 
A  rotary  might  also  furnish  direct  current  for  electric  rail- 
way service. 

Three-Phase  System  for  Railway  Circuits The  con- 
nections of  a  typical  three-phase  distribution  system  for 
furnishing  power  to  electric  railways  are  shown  in  Fig.  147. 
In  the  main  station  are  the  engine-driven  generators,  wound 
for  an  initial  potential  of  6,600  or  13,200  volts.  The  cur- 
rent is  carried  direct  to  the  main  switchboards,  consisting 
of  two  parts,  the  switches  consisting  of  oil  circuit  breakers, 
being  mounted  in  brick  compartments  apart  from  the  op- 
erating panels.  No  high-tension  connections  run  to  the 
operating  panels,  so  that  the  station  attendants  do  not  come 
near  any  of  the  high-tension  apparatus  in  the  operation  of 
the  station. 

The  high-tension  feeders  are  carried  to  alternating  cur- 
rent panels  of  the  substation  switchboard,  which  is  similar 
in  its  safety  features  to  that  installed  in  the  main  station, 
and  thence  to  the  transformers,  which  reduce  the  line 


208       POLYPHASE    APPARATUS    AND    SYSTEMS. 


THREE-PHASE    SYSTEM. 


209 


potential  to  a  suitable  pressure  for  use  with  the  rotary  con- 
verters. The  direct  current  from  these  converters  is 
carried  to  the  third  rail  in  trolley  wire. 

Three-Phase  Lighting  Circuits For  lighting  both  arc 

and  incandescent  by  low-tension  three-phase  distribution, 
two  systems  are  in  use.  The  first  is  known  as  the  single- 
phase,  three-wire  system,  the  general  arrangement  and  con- 


THREE-PHASE  DYNAMO                                                  BUS  BARS 

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SINGLE-PHASE  LIGHTING   FEEDER 

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THREE  WIRE  SYSTEM                                     POWER  W1RE 

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Fig.  148. 

nections  of  which  are  shown  in  diagram  148.  The  second 
system  is  the  three-phase,  four-wire,  which  has  been  de- 
scribed before.  The  connections  of  this  system  for  lighting 
installations  are  shown  in  diagram  149. 

In  the  first  system  the  incandescent  lighting  service  is 
mainly  supplied  from  one  circuit  of  the  three-phase  gene- 
rator, and  the  voltage  is  regulated  with  respect  to  the 
needs  of  the  three-wire  lighting  circuit.  The  primary  cir- 


210   POLYPHASE  APPARATUS  AND  SYSTEMS. 

cults  having  a  small  drop,  make  it  possible  to  lay  out  a 
secondary  network,  having  a  very  superior  regulation. 
When  motors  are  operated  on  this  system,  a  separate  power 
wire  is  required,  the  connections  of  which  are  shown  in 
the  diagram. 

The  advantage  of  the  four-wire,  three-phase  system  is 
that  as  long  as  it  is  balanced,  the  generator  load  remains 


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INSTALLATIONS 


OUAI  i       ifHTINr  HOUSE  TO  HOU 

INSTALLATIONS  THREE-PHASE  MOTOR    DISTRIBUTION  APPROXIMATELY  BALANCED 

Fig.  149. 


balanced.  Another  favorable  feature  is  that  its  outside 
wires  are  available  for  operating  motors.  This  system  is 
not  so  desirable  in  cases  where  the  system  may  be  seri- 
ously unbalanced,  as  by  arc  lamps  or  other  very  inductive 
load  unequally  distributed. 

Three-Phase    Generators The    three-phase    generator 

will  ordinarily  deliver  75%  of  its  rated  capacity,  in  single- 
phase  current,  between  any  two  conductors,  with  the  same 
heating  as  when  delivering  full  three-phase  load.  When 
running  as  a  single-phaser,  or  when  the  load  is  unbalanced 


THREE-PHASE    SYSTEM.  211 

between  the  conductors,  the  potential  differences  of  the 
phases  are  unequal.  The  phase  carrying  the  load  will 
have  one  voltage,  while  one  of  the  disused  or  lightly  loaded 
phases  will  have  a  higher  voltage  and  the  other  a  lower 
voltage.  In  machines  of  good  regulation,  these  differences 
are  small,  and  can  be  readily  taken  care  of  by  regulators  in 
the  feeder  circuits.  These  differences  are  also  smoothed 
out  by  the  use  of  motors  and  synchronous  apparatus  con- 
nected to  the  three  phases.  The  unused  phases  can  be 
loaded  with  other  single-phase  currents,  thus  giving  vary- 
ing degrees  of  unbalancing,  and  increasing  the  load  with 
normal  heating,  inversely  as  the  unbalancing,  up  to  the 
total  three-phase  capacity  of  the  generator.  As  three- 
phase  generators,  for  a  given  output,  are  cheaper  and 
smaller  than  a  single-phaser,  it  will  often  be  found  desir- 
able to  install  a  three-phase  machine  for  single-phase 
working.  Intelligent  and  careful  arrangement  of  the 
feeder  circuits  will  give  the  best  possible  results  as  to 
regulation.  This  is  the  more  easily  obtained  by  the  use 
of  regulators,  which  modern  engineering  demands  shall 
be  installed  in  every  central  station. 


212         POLYPHASE   APPARATUS   AND    SYSTEiMS. 


CHAPTER    XL 
MONOCYCLIC    SYSTEM. 

General The  monocyclic  system  is  essentially  a  single- 
phase  system,  consisting  of  two  wires  in  combination  with 
a  third  auxiliary,  or  teaser  wire  ;  the  main  lines  being  used 
for  supplying  lights,  while  the  third  wire,  which  carries  an 
intermediate,  or  displaced,  current,  is  used,  together  with 
the  main  lines,  for  supplying  power  to  polyphase  motors. 
The  teaser  wire  need  only  be  run  to  the  motors.  Indeed 
the  teaser  wire  need  not  start  from  the  generator,  but  may 
start  from  any  motor,  or  multiple-circuit  apparatus,  of  the 
system.  The  motors  operate  practically  the  same  as  poly- 
phase motors.  As  the  lights  are  connected  to  the  single- 
phase  circuit,  there  is  no  possibility  of  unbalancing.  The 
monocyclic  generator  can  be  loaded  to  its  fullest  extent 
with  either  lights  or  motors,  or  partly  with  lights  and  partly 
with  motors,  in  any  proportion. 

It  has  been  noted  that  the  regulation  of  polyphase  gen- 
erators varies  with  the  inductive  character  of  the  load. 
The  monocyclic  generator,  when  designed  with  shunt  and 
series  excitation,  possesses  the  superior  advantage  of  auto- 
matically compounding  for  all  kinds  of  load.  The  power 
wire  of  the  monocyclic  system  supplies  the  magnetizing 
current  to  the  motors,  which  current  is  returned  over  the 
main  wires,  adding  to  the  magnitude  of  the  current  in 
one  lead,  and  reducing  it  in  the  other.  The  commutating 


MONOCYCLIC    SYSTEM. 


213 


device  is  placed  in  the  main  carrying  the  largest  current. 
As  the  increase  over  the  normal  depends  on  the  motors,  — 
the  inductive  load,  —  the  greater  the  inductive  character 
of  the  load,  the  larger  will  be  the  series-exciting  current. 


Coils 
Armature 


Pig.  15O. 

In  this  way  the  monocyclic  machine  can  be  made  to  give 
perfect  compounding,  on  either  inductive  or  non-inductive 
loads. 

Generator  Armature  Connections The  connections  and 

detail  of  the  monocyclic  generator  armature  are  shown  in 
Fig.  150.  The  armature  coils  are  made  up  of  a  single-phase 


214      POLYPHASE   APPARATUS   AND    SYSTEMS. 

main  winding,  similar  to  the  ordinary  armature  winding  of  a 
single-phase  alternator.  Midway  between  the  main  slots  of 
the  armature  is  a  set  of  smaller  slots,  containing  the  auxili- 
ary winding  of  the  same  cross-section  as  the  main  winding, 
but  of  only  one-quarter  the  number  of  turns.  One  end 

of  the  teaser  coil  is 
connected  to  the  mid- 
dle of  the  main  coil, 
and  the  other  to  a 
third  collector  ring. 
In  this  teaser  coil,  an 

E.M.F.,  in  quadrature  with  that  of  the  main  coil,  is  estab- 
lished, which  is  made  use  of  for  supplying  magnetizing 
current  for  the  operation  of  alternating-current  motors. 

When  the  generator  is  wound  for  2,080  volts,  the  teaser 
coil  has  one-quarter  the  number  of  turns,  and  gives  an 
E'.M.F.  of  520  volts.  The  E.J\I.F.  between  the  terminals 
of  the  main  coil  and  the  free  end  of  the  teaser,  is  the  result- 
ant of  the  E.M.F.  in  the  two  coils,  and  is  shown  in  magni- 
tude and  direction  by  Fig.  151.  The  teaser  may  be  wound 
to  have  86  per  cent  of  the  main  turns,  instead  of  25  per 
cent  ;  in  which  case  the  electromotive  forces  of  the  three 
terminals  are  equal,  and  we  have  a  three-phase  relationship. 
When  the  single-phase  circuit  is  loaded,  the  potential  be- 
tween the  main  does  not  bear  the  same  phase  relationship 
to  the  teaser  terminal  that  it  did  on  open  circuit.  The  cur- 
rent lags  behind  the  impressed  volts,  due  to  the  self-induc- 
tion. The  triangle  of  the  terminal  electromotive  forces  is 
distorted,  so  that,  if  the  main  potential  is  2,080  volts,  that 
between  the  teaser  and  one  main  may  be  1,320  volts,  and 
the  other  800  volts  (Fig.  142).  Loading,  now,  the  teaser 
wire,  produces  a  current  lag,  and  shifts  its  potential  so  that 


MONOCYCLIC    SYSTEM. 


215 


the  triangle  of  E.M.F.  regains  its  normal  shape,  and  the 
electromotive  forces  their  magnitude  and  normal  relation- 
ship (Fig.  143). 

The  wiring  for  monocyclic  circuits,  when  lights  only  are 
supplied,  is  the  same  as  for  single-phase  circuits. 

Systems  of  Distribution. —  In  Fig.  144  is  shown  a  dia- 
gram of  a  monocyclic  system  of  light  and  power  distri- 
bution. 

800  B 


Fig.  152. 


"60 


Fig.  153. 


The  generator,  A,  sends  power  over  the  main  wires,  a 
and  b,  to  transformer,  T,  operating  lights  and  a  small 
single-phase  fan  motor  from  its  secondaries. 

From  the  same  generator  issues  the  teaser,  or  power 
wire,  of  small  cross-section,  shown  in  dotted  lines,  c,  which 
is  carried  to  the  pairs  of  transformers,  D  and  E,  supplying 
motors. 

In  D  is  shown  the  arrangement  of  transformers  suitable 
for  the  operation  of  standard  alternating-current  induction 
motors  from  their  secondaries.  The  two  transformers  are 


2l6   POLYPHASE  APPARATUS  AND  SYSTEMS. 

of  equal  size  and  of  one-half  the  main-line  voltages,  and  are 
connected  with  their  primaries  between  teaser  and  main 
wires,  while  one  of  their  secondaries  is  reversed  with 
regard  to  the  primary,  and  thereby  establishes,  in  the  sec- 
ondary circuit,  a  relation  of  electromotive  forces  suitable 
for  the  operation  of  the  motor. 

In   E  an   arrangement   is   shown,    whereby   lights    and 


Induction 
Motor 


System  of  Secondary  Mains. 


Fig.  154. 


motors,  or,  in  short,  a  whole  three-wire  network,  is  operated 
from  the  transformer  secondaries. 

The  large  or  main  transformer  is  connected  between 
the  main  lines,  a  and  b,  and  is  of  a  size  sufficient  to  supply 
the  total  capacity  of  the  secondary  network.  An  addi- 
tional or  teaser  transformer,  of  one-quarter  the  primary 
main  voltage,  and  of  very  small  size  only,  is  connected  by 
one  terminal  to  the  centre  of  the  main  transformer  coils, 


MONOCYCLIC   SYSTEM.  2I/ 

while  the  other  terminal  connects  with  the  teaser  wire,  c, 
in  the  primary,  and  the  motor  wire  in  the  secondary. 
This  transformer  connection  is  analogous  to  the  connec- 
tion of  main  and  teaser  coil  in  the  generator. 

Supplied  in  this  way,  a  secondary  network  on  the  mono- 
cyclic  system  consists  of  four  conductors,  —  two  main 
conductors,  the  lightning  neutral,  and  the  power  neutral 
or  balance  wire. 

Such  a  secondary  network  can  be  operated  in  the  same 
way  as  a  continuous-current  three- wire  system,  and  offers 
the  essential  advantage  of  saving  the  excessive  amount  of 
copper  in  the  long  feeders,  by  being  applied  from  high- 
potential  lines  through  transformers.  An  unbalancing 
due  to  the  motors  is  not  possible,  and  motors  operated  on 
this  system  do  not  affect  the  lights,  except  in  so  far  as  the 
drop  in  the  mains  is  concerned. 

Arc  lights  can  be  operated  very  satisfactorily  from  the 
monocyclic  system,  and  are  supplied  either  by  compensa- 
tors from  the  secondary  circuits,  or  from  the  primary  cir- 
cuits by  transformers,  as  shown. 

Series  incandescent  lights  can  be  used  for  street  lighting, 
and  are  directly  supplied  from  the  primary  main  lines. 
Where  a  district  has  to  be  supplied,  which  is  too  far 
distant  to  be  reached  directly  by  the  primary  or  gener- 
ator voltage,  step-up  and  step-down  transformers  may  be 
used. 

Transformer  Connections The  various  methods  of  con- 
necting transformers  to  monocyclic  circuits,  and  the  result- 
ant voltages,  are  shown  in  detail  in  Figs.  155  to  157.  It 
will  be  noticed  the  teaser  wire  is  necessary  only  where 
motors  are  used,  the  lights  being  connected  on  the  single- 
phase  system.  Fig.  155  shows  the  detailed  connections 


2l8       POLYPHASE   APPARATUS   AND    SYSTEMS. 

and  standard  voltages  in  a  system  for  operating  lights  and 
motors  from  the  same  transformers. 

The  three-phase  relationship  for  operating  power  appa- 
ratus may  be  obtained  by  the  transformer  connection,  as 


Generator 


TeazerWire 


Fig-.  155. 

shown.  The  primaries  of  the  transformers,  which  are  of 
different  capacities,  are  connected  and  wound  to  produce 
the  exact  E.M.F.  relationship  of  the  generators.  The 
large  transformer  is  connected  across  the  main  circuit, 
while  the  supplementary  transformer  is  connected  to  the 
middle  of  the  large  transformer  and  to  the  teaser  wire. 
The  ratio  of  transformation  c  f  each  transformer  is  selected 


MONOCYCLIC   SYSTEM. 


219 


so  that  the  secondary  E.M.F.  of  the  smaller  transformer 
is  about  82  per  cent  of  that  of  the  larger.  This  gives  a 
slightly  lop-sided  three-phase  relationship,  from  which 
motors  of  1 10  volts  and  lights  of  1 1 5  volts  can  be  operated. 
Of  course  an  exact  three-phase  relationship  can  be  ob- 
tained by  raising  the  E.M,F.  of  the  smaller  transformer  to 
86  per  cent.  The  smaller  transformer  should  be  about 
one-third  the  capacity  of  the  motor,  or,  if  a  number  of 


2  Transform 


Fig.  156. 

the  motors  be  used,  about  one-fourth   the  aggregate  ca- 
pacity. 

A  beautiful  illustration  of  the  resultant  three-phase  rela- 
tionship is  the  use  of  two  transformers  with  a  monocyclic 
generator,  the  secondary  of  one  being  reversed  (Fig.  156). 
The  diagram  of  E.M.F.  (Fig.  157)  shows  the  effect  of 
reversing  the  secondary.  The  three  motor  wires  are  con- 
nected to  A,C,D]  the  difference  in  phase  being  nearly, 


220       POLYPHASE   APPARATUS   AND    SYSTEMS. 

though  not  quite,  60°.  The  secondary  circuits,  from  such 
an  arrangement,  may  be  considered  as  practically  the  same, 
and  have  all  the  advantages  of  a  straight  three-phase 
system. 

A  third  method  of  obtaining  the  proper  phase  relation- 
ship for  motor  work  is  shown  in  diagram  (Fig.  158).     In 


Fig.  157, 

this  arrangement  only  one  transformer  is  required,  having 
half  the  capacity  required  for  the  other  methods  of  operat- 
ing motors.  The  primary  is  connected  between  one  of  the 
mains  and  the  teaser  wire.  The  secondary  coil  is  in  series 
with  the  primary,  and  has  the  same  number  of  turns  ;  the 
ratio  of  transformation  being,  therefore,  i  to  I.  As  the 
primary  current  of  a  transformer  differs  from  the  second- 
ary current  180°  in  phase,  one  leg  of  the  circuit  is  naturally 
inverted,  changing  the  relation  of  the  phases  from  mono- 
cyclic  to  three-phase. 

This  arrangement  is  especially  suited  for  the  operation 
of  large  motors,  as  the  cost  of  transformers  is  reduced 
one-half.  The  voltage  of  the  motors  is  fixed  at  approxi- 
mately one-half  the  generator  voltage. 

The  most  common  and  convenient  connection  of  trans- 
formers, when  motors  alone  are  to  be  operated  from  a 
monocyclic  circuit,  is  that  shown  in  Fig.  156.  The  ratio 
of  transformation  of  the  transformer  here  shown  is  about 


MONOCYCLIC   SYSTEM. 


221 


9  to  I.  In  the  operation  of  motors  from  1,040  volt  mono- 
cylic  circuits,  transformers  of  the  ratio  of  4-*-  to  i  must  be 
used. 

Monocyclic  Motors.  —  The  three-phase  induction  motor  is 
the  most  suitable  for  use  on  monocyclic  circuits.  The  two- 
phase  motor  can  be  used  with  an  increase  in  the  number 
of  turns  of  the  teaser  winding,  but  at  the  expense  of  the 
output  of  the  generator,  considered  solely  as  a  single-phase 
machine.  The  performance  of  the  three-phase  motor,  con- 


Fig.  158. 

nected  in  a  monocyclic  system,  in  regard  to  efficiency, 
torque,  and  power  factor,  is  essentially  the  same  as  on  a 
straight  three-phase  system.  The  flow  of  current  is  dif- 
ferent in  the  three  conductors,  the  teaser  wire  carrying 
mainly  the  magnetizing  current.  A  monocyclic  motor  of 
the  induction  type  can  be  used,  the  windings  of  which  are 
exact  reproductions  of  the  generator  windings, — i.e.,  are 
two  in  number  ;  one  having  25  per  cent  the  turns  of  the 
other,  and  connected  to  the  middle  of  the  large  coil.  Such 
a  motor  can  be  run  from  two  transformers ;  or,  if  of  a  size 
permitting  it  to  be  wound  for  the  high  voltage  of  the  main 


222   POLYPHASE  APPARATUS  AND  SYSTEMS. 

circuits,  direct  from  the  mains.  While  the  monocyclic 
generator  can  be  fully  loaded  with  induction  motors,  which 
interchange  the  magnetizing  current  by  means  of  the  teaser, 
it  is  not  advisable  to  run  one  large  induction  motor  of  a 
size  approximating  that  of  the  generator.  In  special  cases, 


Fig".  159. 

where  the  motor  need  not  have  a  large  starting  torque,  this 
arrangement  is  permissible. 

Synchronous  motors  on  a  monocyclic  system  need  not 
be  operated  from  reversed  transformers,  but  can  be  run 
direct  from  the  generators,  provided  they  are  identical  with 
them.  They  have  little  starting-torque,  and  require  an  ex- 
traneous source  of  power  to  bring  them  to '"synchronism. 


MONOCYCLIC  SYSTEM.  223 

Measurement  of  Power.  —  The  power  supplied  to  lights 
and  other  single-phase  current-consuming  devices,  is  meas- 
ured by  the  standard  forms  of  wattmeters.  On  account 
of  the  uncertain  flow  of  current  in  the  motor  connections, 
special  connections  are  necessary.  Fig.  159  shows  a  re- 
cording wattmeter  connected  to  measure  the  power  deliv- 
ered to  motors.  One  of  the  field  coils,  I),  is  connected  in 
the  common  return  D  ;  the  other  coil,  E,  in  the  main  A,  or 
perhaps  in  the  main  C.  If  the  motor  is  loaded  and  the 
meter  speeds  up,  the  connections,  as  shown,  are  right.  If 
the  meter  speed  diminishes  with  increasing  motor  load,  the 
field  coil  E  should  be  connected -in  the  main  C.  This 
meter  will  be  found  to  give  fairly  accurate  results.  At 
high  loads  the  reading  will  be  found  slightly  high,  but  not 
sufficiently  to  be  commercially  objectionable. 

The  standard  form  of  induction  meter  is  generally  ap- 
plicable to  monocyclic  circuits  and  gives  accurate  results. 


224       POLYPHASE   APPARATUS   AND    SYSTEMS. 


CHAPTER   XII. 
CHOICE  OF  FREQUENCY. 

High  Frequencies. — In  designing  a  plant  for  the  distri- 
bution of  light  and  power  by  polyphase  currents,  one  of  the 
first  considerations  that  presents  itself,  is  whether  the  ap- 
paratus shall  be  of  high  or  low  frequency.  By  high  fre- 
quency is  generally  understood  to  mean  one  of  over  60 
cycles  per  second,  or  7,200  alternations  per  minute.  Sixty 
cycles  and  less  are  considered  low  frequencies.  Until 
quite  recently,  the  frequencies  generally  employed  in  the 
United  States  were  125  and  133  cycles,  or  15,000  and 
16,000  alternations.  Abroad,  the  commercial  frequencies 
were  somewhat  lower,  varying  from  80  to  100  cycles.  The 
adherence  to  a  high  frequency  in  this  country  for  over  ten 
years  has  resulted  in  an  investment  of  millions  of  dollars  in 
this  particular  type  of  apparatus,  and  has  made  the  intro- 
duction of  new  types  of  lower  frequency  into  old  and  exist- 
ing central  stations  somewhat  difficult,  even  when  evident 
economy  and  advantage  have  been  shown  to  follow  upon 
such  introduction. 

The  tendency  of  modern  alternating-current  practice  is 
in  the  direction  of  low  frequencies,  and  in  the  organization 
of  a  new  plant,  the  problem,  in  nine  cases  out  of  ten,  is 
confined  to  the  selection  of  a  frequency  of  60  cycles  or 
under. 

There  are  frequently   strong   reasons  for  retaining    or 


CHOICE   OF   FREQUENCY.  225 

adopting  125  or  133  cycles.  One  of  these  has  been  men- 
tioned above.  The  change  from  125  cycles  to  a  lower 
frequency  necessitates  a  complete  revamping  of  the  instal- 
lation, and,  with  the  exception  of  the  small  sizes,  the  trans- 
formers must  be  replaced.  Again,  when  a  low  first-cost 
of  a  plant  is  considered  of  more  importance  than  a  possible 
ultimate  saving  of  operating  expenses,  and  a  more  satisfac- 
tory service,  a  high  frequency  will  be  used.  The  gener- 
ators are  cheaper,  as  they  run  at  a  higher  speed.  The 
transformers  are  also  smaller  and  cheaper. 

One  of  the  drawbacks  to  the  use  of  high  frequencies, 
especially  in  the  transmission  of  power  over  lines  of  consid- 
erable length,  is  the  drop  of  voltage  due  to  the  reactance  of 
the  line,  which  increases  with  the  frequency.  For  illustra- 
tion :  the  reactance  of  1,000  feet  of  No.  i  wire,  at  25 
cycles,  is  .0486  ohms,  and  at  125  cycles,  .243  ohms.* 

By  reducing  the  frequency  from  125  cycles  to  25  cycles, 
in  the  above  case,  the  voltage  drop,  due  to  the  reactance 
and  resistance,  is  reduced  almost  one-half.  With  heavier 
conductors  and  higher  frequencies,  the  difference  is  still 
more  noticeable.  The  effect  of  frequency  on  the  volt- 
age drop  in  transmission  lines  is  treated  at  further  length 
in  Chapter  XIV.  In  lighting  plants  employing  large 
conductors,  on  account  of  the  varying  power-factors  due 
to  changing  character  of  load,  the  irregularity  in  volt- 
ages at  high  frequencies  may  become  quite  marked.  As 
we  have  seen,  this  voltage  drop  is  not  all  energy  loss,  this 
loss  being  only  proportional  to  the  energy  component  of 
the  total  drop. 

Other  disadvantages  in  the  use  of  high  frequencies  are 
the  speed  at  which  both  generators  and  motors  must  run 

*  See  Table  of  Line  Constants  for  Power  Transmission,  page  224. 


226   POLYPHASE  APPARATUS  AND  SYSTEMS. 

in  order  not  to  unduly  increase  the  number  of  poles,  and 
the  difficulty  in  connection  with  engine  regulation,  when  a 
number  of  generators  are  direct  driven,  and  operated  in 
parallel.  High-frequency  as  well  as  low-frequency  induc- 
tion motors  operate  better  at  high  speeds,  but  these  are 
undesirable  from  both  mechanical  and  commercial  stand- 
points. On  the  other  hand,  high-frequency  induction 
motors  of  reduced  speeds  have,  as  a  rule,  low  power-fac- 
tors. The  high-frequency  induction  motor  was  introduced 
to  meet  the  demand  for  motors  of  small  power  on  high 
frequency  circuits.  With  the  system  on  which  it  is  at 
present  operated,  it  may  in  time  become  a  thing  of  the 
past. 

To  sum  up  :  High  frequencies  permit  the  use  of  cheap 
generators  and  transformers,  and,  in  addition,  the  simple 
and  satisfactory  operation  of  incandescent  and  arc  lamps 
and  synchronous  motors.  They  have  the  disadvantage  of 
increasing  the  voltage  drop  and  idle  currents  of  circuits, 
with  consequent  bad  regulation  and  the  heating  of  the 
generator  at  light  loads,  of  not  permitting  the  parallel 
operation  of  direct-connected  machines  of  low  speed,  and 
the  further  disadvantage,  that  induction  motors  must  either 
run  at  excessive  speeds,  or  with  poor  power-factors.  Syn- 
chronous motors  will  not  start  with  the  same  vigor  as  on 
lower  frequencies. 

Low  Frequencies.  —  Up  to  the  present  time  no  arc 
lamp  has  been  made  that  will  operate  with  entire  satis- 
faction on  frequencies  of  less  than  40  cycles.  Incandes- 
cent lamps  cannot  be  used  to  advantage  on  frequencies 
less  than  30  cycles.  Low-voltage  incandescent  lamps  show 
no  nicker  ;  but  the  effect  of  fatiguing  the  eye  is  noticeable 
at  25  cycles,  especially  in  high  voltage  lamps. 


CHOICE    OF    FREQUENCY.  22/ 

Transformers  are  somewhat  bulkier,  more  expensive,  and 
slightly  less  efficient  at  low  frequencies.  Induction  motors, 
while  likewise  larger  and  more  expensive,  as  a  rule  can  be 
built  with  equal,  if  not  better,  power  factors,  and  at  con- 
venient and  commercial  speeds.  Rotary  converters  can  be 
successfully  designed  for  60  cycles.  The  speed  is  high, 
however,  and  the  best  mechanical  and  electrical  results  are 
obtained  at  frequencies  under  40  cycles.  The  largest  use 
of  miscellaneous  power  by  rotary  converters  is  at  Niagara, 
where  a  frequency  of  25  cycles  is  employed.  The  largest 
use  of  power  for  electric  railway  work  by  rotary  converters 
is  at  St.  Anthony  Falls,  Minneapolis  ;  the  frequency  here 
being  approximately  35  cycles. 

The  Niagara  plant  is  essentially  a  power  plant.  The 
use  of  current  for  both  arc  and  incandescent  lighting  is  of 
no  great  importance.  The  power,  electrically  generated 
on  a  scale  never  before  attempted,  is  used  locally  in  a  great 
variety  of  processes,  and  is  delivered  in  a  form  most  suit- 
able for  its  diverse  uses.  Power  by  the  direct-current 
system,  while  convenient  for  some  particular  operations, 
would  not  answer  equally  well  all  requirements  at  Niagara, 
and  would  be  unsuitable  for  long-distance  transmission. 
A  high-frequency  system  would  restrict  the  use  of  motors 
and  rotary  converters,  and  the  transmission  of  power  over 
very  long  distances.  Sixty  and  40  cycles,  however,  permit- 
ting the  general  use  of  lighting  apparatus,  do  not  give 
the  best  results  with  rotary  converters  of  large  output. 
The  operation  of  2  5 -cycle  rotary  converters,  on  the  scale 
employed  at  Niagara,  shows  that,  for  the  purely  power 
conditions  there  existing,  this  frequency  was  wisely  chosen. 

A  frequency  of  25  cycles  is  also  used  by  the  Brook- 
lyn Edison  Illuminating  Company  in  the  extension  of 


228   POLYPHASE  APPARATUS  AND  SYSTEMS. 

their  plant.  Four  thousand  H.P.  are  transmitted  within 
an  area  covering  75  miles,  to  various  substations,  where  25- 
cycle  rotary  converters  are  stationed.  These  deliver  1 1 5 
volt  direct-current  into  Edison  three-wire  mains.  The 
Chicago  Edison  Company  use  a  somewhat  similar  system 
of  distribution  and  the  same  frequency. 

For  the  general  conditions  of  a  power  plant,  supplying 
alternating  current  for  induction  motors  and  lighting,  and 
making  a  specialty  of  furnishing  direct  current  on  a  large 
scale,  at  some  distance  from  the  generating  plant,  a  fre- 
quency of  35  to  40  cycles  will  be  found  suitable. 

The  frequency  of  60  cycles,  or  7,200  alternations  per 
minute,  has  come  into  extensive  use.  It  has  the  advan- 
tage of  considerably  reducing  line  reactance  and  the  idle 
currents  present  in  lighting  systems  of  higher  frequencies. 
It  is  adapted  for  the  most  economical  results  in  a  general 
distributing  system  of  lights  and  motors.  On  account  of 
the  good  regulation  possible  with  this  frequency,  the  high- 
est economy  lamps  can  be  used.  Sixty-cycle  motors  are 
excellent  in  respect  to  efficiency  and  power  factor,  and  run 
at  commercial  speeds.  Both  motors  and  transformers  are 
reasonable  in  cost. 

When  the  generating  units  are  direct-connected  to 
engines  of  extremely  slow  speed  and  operated  in  parallel, 
a  frequency  of  60  cycles  will  be  found  to  be  not  desirable. 
As  explained  in  Chapter  III.,  the  permissible  variation  in 
rotative  speed  is  not  so  great  as  with  lowrer-frequency  gen- 
erating units. 

Choice  of  Frequency.  —  It  is  impossible  to  make  more 
than  the  most  general  application  of  the  foregoing 
remarks.  Each  particular  case  must  be  studied  in  the 
light  of  its  special  conditions,  before  an  intelligent  decision 


CHOICE    OF    FREQUENCY.  229 

can  be  made  as  to  the  proper  frequency  to  employ.  At 
the  risk  of  repetition,  the  following  general  recommenda- 
tions are  suggested  as  embodying  the  latest  and  standard 
practice  : 

For  local  lighting  systems  with  incidental  demand  for 
power  in  small  units,  where  old  transformers  have  to  be 
retained,  and  where  a  cheap  plant  is  of  first  consideration, 
a  high  frequency  may  be  used,  but  should  be  discouraged 
as  much  as  possible. 

For  general  transmission  and  distribution  for  lighting 
and  power  purposes,  conditions  which  accompany  the  ma- 
jority of  alternating-current  propositions,  a  standard  fre- 
quency of  60  cycles  can  be  used  to  advantage. 

In  power  and  lighting  plants,  --  where  arc  lighting  is  of 
secondary  consideration,  --supplying  current  to  induction 
motors,  as  in  mill  work,  and  to  rotary  converters,  as  in 
long-distance  railway-transmission  work,  where  the  gen- 
erators are  direct  driven  by  engines,  and  finally,  for 
very  long  transmissions  of  power,  a  frequency  of  40 
cycles,  or  thereabouts,  may  be  used.  This  is  a  good, 
all-round  frequency,  and  is  coming  into  more  general 
use  in  this  country.  It  is  much  employed  abroad. 

For  exclusively  power  plants,  where  lighting  is  of  no 
importance  whatsoever,  and  where  rotary  converters  and 
motors  of  large  size  or  slow  speed  are  to  be  supplied,  a 
frequency  of  25  to  30  cycles  may  be  used. 

Notwithstanding  the  opportunity  for  the  careful  exercise 
of  judgment  in  selecting  a  proper  frequency,  almost  equally 
good  results  can  be  obtained  with  widely  different  fre- 
quencies. As  an  illustration,  the  Brooklyn  Edison  Com- 
pany have  adopted  25  cycles  for  their  power  and  rotary 
converter  work.  The  Boston  Edison  Company  obtain 


230   POLYPHASE  APPARATUS  AND  SYSTEMS. 

practically  the  same  results,  using  a  frequency  of  60 
cycles.  Power  is  transmitted  over  the  150  miles  of  line 
of  the  Bay  Counties  Company  in  California  at  a  frequency 
of  60  cycles.  The  100  mile  transmission  of  the  Govern- 
ment of  Mysore,  India,  is  accomplished  at  a  frequency  of 
25  cycles. 


RELATIVE    WEIGHTS    OF    COPPER.  231 


CHAPTER    XIII. 

RELATIVE    WEIGHTS  OF   COPPER    FOR   VARIOUS 
SYSTEMS. 

As  the  transmission  and  distribution  of  power  often 
involves  a  large  outlay  for  copper  conductors,  it  is  most 
important  to  ascertain  what  system  and  what  combination 
of  conductors  will  give  the  most  economical  results.  In 
making  any  comparison  between  the  copper  efficiencies  of 
the  various  systems,  the  proper  basis  of  comparison  is 
equality  of  voltage. 

The  amount  of  copper  required  for  transmitting  a  given 
power  at  a  fixed  percentage  loss  is  found  by  the  rule  that 
the  weight  of  copper  varies  inversely  as  the  square  of  the 
voltage. 

The  voltage  of  an  alternating  circuit,  as  measured  by 
the  ordinary  commercial  instruments,  —  i.e.,  the  effective 
voltage,  —  is  about  40  per  cent  less  than  the  maximum 
value  of  the  E.M.F.  wave.  It  is  this  maximum  value  that 
must  be  considered  in  determining  the  break-down  point 
of  insulation  and  the  highest  voltage  that  can  be  used 
commercially,  as  in  the  long-distance  transmission  of 
power.  On  the  other  hand,  when  the  maximum  voltage 
of  a  circuit  is  within  the  limit  of  safe  insulation  strain,  the 
effective  voltage  carries  no  limitation  with  it. 

The  comparison,  then,  of  the  various  systems,  to  deter- 
mine the  most  economical  method  of  transmission,  will  be 


232   POLYPHASE  APPARATUS  AND  SYSTEMS. 

either  on  the  basis  of  maximum  potential,  as  in  the  case  of 
long  transmission  lines,  or  on  the  basis  of  effective  or  min- 
imum potential,  as  in  the  case  of  low-potential  distributions 
by  secondary  mains. 

Figs.  1 60  to  1 66  show  the  standard  systems  of  alternat- 
ing-current distribution  and  the  various  combinations  of 

o 

conductors  in  general  use.  The  name  of  each  system  is 
given,  and  also  the  relative  amount  of  copper  required. 

The  percentual  amount  of  copper  required  by  the  single- 
phase  system,  which  is  here  taken  as  the  standard  of  com- 
parison for  the  other  systems  and  combinations,  is  illus- 
trated by  diagram  (Fig.  160).  The  single-phase  three- 
wire  system  is  shown  in  Fig.  161.  If  the  voltage  of  the 
two-wire  system  is  c,  the  potential  between  the  two  outside 
wires  is  2c.  Applying  the  rule  that  the  amount  of  copper 
is  inversely  as  the  square  of  the  voltage,  only  i  the  copper 
would  be  needed,  if  the  neutral  should  have  no  cross-sec- 
tion, or  the  return  conductor  be  dispensed  with,  as  might 
be  done  in  the  case  of  a  perfect  balance.  If  the  neutral  is 
given  a  cross-section  equal  to  one  of  the  outside  wires,  the 
total  copper  in  the  three-wire  single-phase  system  is  37.5 
per  cent  that  of  the  two-wire  single-phase  system.  With 
a  neutral  -J-  and  J  the  cross-section  of  the  outside  wires,  the 
total  copper  is  31.25  per  cent  and  29.15  per  cent  respec- 
tively of  our  standard  system.  In  a  four-wire  system  the 
voltage  between  outside  wires  is  3^,  and,  under  perfect  bal- 
ance, ^  the  amount  of  copper  would  be  required.  When 
the  neutral  and  outside  wires  are  of  equal  size,  the  copper 
must  be  increased  to  22.2  per  cent.  In  like  manner  the 
copper  in  the  five-wire  system,  with  neutrals  of  full  cross- 
section,  is  15.62  per  cent,  and  the  same  system,  with  neu- 
trals of  %  the  area  of  the  neutral  wires,  requiring  only  10.93 


RELATIVE    WEIGHTS    OF   COPPER. 


233 


SYSTEM 


Single  Phase 
2  Wire 


Single  Phase 
3   Wire 


Two  Phase 
4  Wire 


Two  Phase 
3  Wire 


WIRING  CONNECTIONS 


Fig.  160. 


Fig.  161. 


Three  Phase    ^|r       1 
3  Wire  < 


Three  Phase 
4  Wire 


Monocyolic 


Fig.  162. 


Fig.  163. 


Fig.  164. 


Fig.  165. 


Fig.  166. 


PER  CENT. 
COPPER 

100. 


37.5 


100. 


j  145.7 
1   72.9 


75. 


33.3 


100. 
125. 
150. 


DIAGRAM 

I 

r 

4- 


L. 

A 


per  cent  of  the  copper  of  the  simple  alternating  circuit. 
These  results  are  the  same  whether  the  comparison  is 
made  on  the  basis  of  maximum  potential,  or  on  the  basis  of 
effective  or  minimum  potential 

In  the  three-phase  system,  the  copper  required  for  cer« 


234   POLYPHASE  APPARATUS  AND  SYSTEMS. 

tain  given  conditions  is  75  per  cent  of  the  copper  used  in 
the  single-phase  system.  The  comparison  between  poly- 
phase systems  can  best  be  made  by  resolving  each  into  as 
many  single-phase  systems  as  it  has  phases.  The  three- 
phase  system  consists  of  three  single  circuits  with  a  common 
ground,  or,  what  is  the  same,  with  no  return  ;  for  the  total 
current  to  and  from  the  centre  is  zero.  If  the  A  or  line  vol- 
tage is  e  (Fig.  164),  the  pressure  or  volts  between  any  wire 

f> 

and  the  juncture  is  — =.     The  single-phase  system,  having  a 

V3 
line  voltage  e,  can  also  be  converted  into  two  single  circuits 

of  voltage  -  (Fig.  162).  As  the  weight  of  copper  in  each 
system  is  inversely  as  the  square  of  the  voltage,  we  have  : 

/  2  \2     /  VI  \2 

(-1:1-  -1=4:3- —  or  the  relative  amounts  of  copper, 

for  the  single-phase  and  the  three-phase  systems,  are  100 
per  cent  and  75  per  cent.  Now  the  two-phase  four-wire 
system,  consisting  of  two  single-phase  systems,  is  placed, 
in  respect  to  the  amount  of  copper  required  for  equal  con- 
ditions, in  the  same  position  as  the  single-phase  system. 
Therefore  the  relative  amounts  of  copper  for  the  two-phase 
and  three-phase  systems  are  100  per  cent  and  75  per  cent. 
Fig.  163  illustrates  the  two-phase  three-wire  distribu- 
tion, two  of  the  wires  of  the  four-wire  system  being  replaced 
by  one  of  full  cross-section.  The  voltage  between  the 
two  outside  conductors  is  now  raised  to  \f  2c  =  1.412  c,  e 
being  the  potential  between  the  conductors  of  either  phase. 
The  amount  of  copper  required,  when  compared  with  the 
single-phase  system,  will  differ  considerably  according  as 
the  comparison  is  based  on  the  highest  voltage  permissible 
for  any  given  distribution,  or  on  the  minimum  voltage  for 


RELATIVE    WEIGHTS    OF   COPPER.  235 

low-tension  service.  If  e  is  the  maximum  voltage,  that 
can  be  used  on  account  of  the  insulation  strain,  or  for  any 
other  reason,  the  pressure  between  the  other  conductors 
of  the  two-phase  three-wire  system  must  be  reduced  to 

The  weight  of  copper  required  under  this  condition 

V2 

is  145.7  per  cent  of  the  single-phase  copper.  If  the  limit- 
ing conditions  of  voltage  do  not  exist,  a  comparison  of  the 
relative  weights  of  copper  can  be  made  with  the  effective 
voltage  of  either  phase  as  a  basis,  —  i.e.,  on  a  basis  of  the 
minimum  voltage.  In  this  case  we  find  a  relative  saving 
over  the  single-phase  circuit  of  about  27  per  cent,  the 
actual  amount  of  copper  being  72.9  per  cent  of  the  single- 
phase  conductors. 

Fig.  165  shows  the  connections  of  the  three-phase  four- 
wire  system.  When  the  fourth  wire,  or  neutral,  is  of  full 
cross-section,  the  copper  required  is  33*  per  cent  of  the 
single-phase  system.  By  making  the  neutral  one-half  the 
cross-section  of  the  main  conductors,  the  copper  weight  is 
reduced  to  29.17  per  cent.  This  arrangement  is  only  used 
for  secondary  systems  of  distribution,  as  described  before. 
The  comparison  with  any  other  system  is,  therefore,  made 
only  on  a  basis  of  equality  between  phases  of  minimum 
voltage. 

The  monocyclic  system  (Fig.  166)  is  treated  as  a  single- 
phase  system  in  the  calculation  of  its  lighting  circuits. 
When  motors  are  connected  to  the  circuit,  the  single-phase 
copper  is  increased  proportionally  to  the  motor  load,  and 
by  the  teaser  wire.  The  rule  governing  the  size  of  the 
teaser  wire  is,  that  its  cross-section  should  bear  the  same 
relation  to  that  of  the  main  wires  that  the  motor  load  does 
to  the  total  load. 


236      POLYPHASE   APPARATUS   AND    SYSTEMS. 


If  the  teaser  is  made  of  a  cross-section  equal  to  one  of 
the  main  conductors,  the  total  weight  of  copper  is  150  per 
cent  of  that  in  a  single-phase  circuit  of  equal  voltage  and 
power.  If  the  load  is  equally  divided  between  motors  and 
lights,  the  teaser  has  a  cross-section  of  one-half  the  main 
conductors,  and  the  total  copper  is  125  per  cent  of  the 
single-phase  copper.  The  main  circuit  can  be  connected 
as  a  three-,  four-,  or  five-wire  system.  The  amount  of  cop- 
per required  is  found  by  adding  the  proportionate  weight 
of  the  teaser  wire.  In  this  way  a  three-wire  monocyclic 
circuit,  neutral  one-half  cross-section,  loaded  one-half  with 
lights,  one-half  with  motors,  will  require  39  per  cent  of  the 
copper  of  the  single-phase  system. 

The  following  Tables  are  compiled  from  data  in  Mr. 
Steinmetz's  valuable  work,  "  Alternating-Current  Phenom- 
ena." The  first  Table  gives  the  relative  copper  efficiencies 
of  various  systems,  when  the  comparison  is  on  the  basis  of 
equality  of  minimum  difference  of  potential.  The  second 
gives  the  relative  weights,  when  the  comparison  is  based 
on  the  equality  of  the  maximum  potential  difference  in 
the  system. 

Amount  of  copper  required  for  transmission  at  a  given  loss,  based 
on  minimum  potential. 


..    SYSTEM. 

No.  OF  WIRES. 

PER  CENT 

COPPER. 

Single-phaise    ."  .              

2 

IOO 

Single-phase 

•77  e 

Two-phase,  common  return  .... 
Two-phase   ... 

3 

4 

J/O 
72.9 
IOO 

Three-phase     
Three-phase,  neutral  full  section  .     .     . 
Three-phase,  neutral  one-half  section    . 

3 

4 
4 

75- 
33-3 
29.17 

RELATIVE    WEIGHTS    OF   COPPER. 


237 


Amount  of  copper  required  for  transmission  at  a  given  loss,  based 
on  maximum  difference  of  potential. 


SYSTEM. 

No.  OF  WIRES. 

PER  CENT 

COPPER. 

Single-phase     

2 

TOO 

Two-phase,  with  common  return  .     .     . 
Two-phase  

3 

4" 

145-7 
IOO 

Three-phase     

•3 

7C 

Direct  Current     

2 

SO- 

It  will  be  seen  that  the  direct-current  system  requires 
only  50  per  cent  of  the  copper  in  the  single-phase  system 
when  used  in  long-distance  transmission  of  power.  The 
advantage  is  not  so  evident,  however  ;  for,  as  Mr.  Stein- 
metz  has  pointed  out,  in  addition. to  the  electrostatic  stress, 
an  electrolytic  effect  is  set  up,  which  does  not  exist  to  the 
same  extent  in  alternating  currents.  The  difficulties  at- 
tending the  utilization  of  direct  current  of  high  tension, 
are  such  that,  with  the  exception  of  one  or  two  special 
and  isolated  cases,  its  employment  in  the  long  distance 
transmission  of  power  has  not  been  seriously  considered. 


238        POLYPHASE    APPARATUS    AND    SYSTEMS. 


CHAPTER  XIV. 
CALCULATION   OF  TRANSMISSION    LINES. 

Line  Constants.  —  As  explained  in  Chapter  I.,  the  drop 
of  voltage  in  an  alternating-current  circuit  will  vary  with 
the  resistance  and  the  reactance  of  the  circuit,  and  with 
the  character  of  the  load.  In  the  table,  "  Line  Constants 
for  Power  Transmission,"  taken  from  a  publication  of  the 
General  Electric  Company,  the  relation  of  reactance  to 
resistance  is  shown  for  a  number  of  frequencies,  and  for 
the  sizes  of  conductors  ordinarily  used  in  power  transmis- 
sions, and  also  other  constants  of  transmission  circuits, 
such  as  capacity  inductance  and  charging  current.  The 
following  explanations  will  serve  to  make  the  table  clear : 

The  E.M.F.  consumed  by  resistance  r,  of  the  line,  is  =  Ir, 
and  in  phase  with  the  current  I. 

The  E.M.F.  consumed  by  the  reactance,  S,  of  the  line,  is  = 
IS,  and  in  quadrature  with  the  current  /. 

The  E.M.F.  consumed  in  the  line,  is  neither  Ir  nor  IS,  but 
depends  upon  the  phase  relation  of  current  in  the  receiving 
circuit. 

The  loss  of  energy  in  the  line  is  =  /V,  hence  does  not  de- 
pend upon  the  reactance,  but  only  upon  the  resistance. 

Two  wires  in  parallel  have  the  same  resistance,  and  about 
half  the  reactance  (if  strung  on  separate  insulators  and  inter- 
mixed) of  a  single  wire  of  double  cross-section.  Thus  replacing 
one  No.  oooo  wire  by  two  No.  o  wires,  the  resistance,  weight 


CALCULATION    OF   TRANSMISSION    LINES.      239 

of  copper,  etc.,  will  remain  the  same,  but  the  reactance  will  be 
reduced  practically  to  half,  so  where  lower  reactance  is  desired, 
the  use  of  several  conductors  strung  on  independent  insulators 
and  intermixed  is  advisable. 

The  values  given  for  Z,  C,  i  ,  and  S  are  calculated  for  sine- 
waves  of  current  and  E.M.F. 

This  table  will  be  found  most  convenient  for  determin- 
ing the  characteristics  of  transmission  circuits  when  the 
size  of  conductor  has  been  fixed.  The  conductors  are  as- 
sumed to  be  separated  by  a  distance  of  18  inches. 

Let  us  take,  as  an  example,  a  case  where  it  is  required 
to  deliver,  by  the  three-phase  60  cycle  system,  2,000  H.P. 
at  the  secondary  terminals  of  the  step-down  transformers, 
over  a  circuit  1  1  miles  in  length.  It  is  further  assumed 
that  the  voltage  at  the  receiving  end  is  10,000,  and  the 
total  energy  loss  in  transmission  from  the  generator  ter- 
minals is  not  to  exceed  15  per  cent.  The  power  is  to  be 
used  for  a'  mixed  system  of  lights  and  induction  motors, 
the  latter  forming  most  of  the  load.  The  power  factor  of 
the  system  at  the  receiving  end  will  be  approximately  85 
per  cent.  We  can  assume  that  — 

The  transformers  have  an  efficiency  of  97^  per  cent. 
The  copper  loss  in  each  being  i  per  cent. 
The  core  or  hysteresis  loss,  i-J-  per  cent 
The  reactance  can  be  taken  as  3^  per  cent. 
And  the  magnetizing  current  4  per  cent. 
The  voltage  between  any  branch  of  the  circuit  and  the 
common  centre  of  the  system  is 

5.775. 


V3 
The  energy  delivered  by  each  branch  is 

.W.^500  K.W. 


240       POLYPHASE  APPARATUS  AND    SYSTEMS. 
The  apparent  energy  delivered  by  each  branch  is 


1.1.  U        '          5^8,000 

The    total    current    in    each    branch    is  -  -  =  102 

5>775 
amperes. 

The  /J?.  drop  in  each  branch  is  10  per  cent  of  5,775  = 

577.5  volts. 

^77  "5 
The  total  resistance  R  =  ^—^  =  5.66  ohms. 

The  resistance  of  one  mile  is  -    -  =  .514  ohms,  which 

is  very  nearly  the  resistance  of  No.  o  wire.  Three  No.  o 
wires,  therefore,  will  carry  2,000  H.P.  a  distance  of  n 
miles  with  a  waste  of  energy  of  10  per  cent,  the  pressure 
at  the  receiving  end  being  10,000  volts  and  power  factor 
85  per  cent. 

By  referring  to  the  table  the  characteristics  of  this 
transmission  line  are  readily  obtained.  The  reactance  of 
eleven  miles  of  single  conductor  is  seen  to  be  6.62  ohms 
at  the  frequency  employed.  The  inductance,  or  what  is 
the  same  thing,  the  coefficient  of  self-induction  of  the  line, 
is  17.6  Millihenrys.  The  charging  current  of  each  line  for 
the  eleven  miles,  with  the  given  voltage  and  frequency,  is 
found  to  be  .4  of  an  ampere. 

It  is  interesting  to  know  what  the  impressed  or  genera- 
tor E.M.F.  and  the  distribution  of  current  will  be,  in  this 
case,  when  the  plant  is  fully  loaded.  For  this  investiga- 
tion, the  entire  system  may  be  reduced  to  a  uniform  vol- 
tage, by  multiplying  the  voltages  by  the  various  ratios  of 
transformation,  thus  bringing  both  the  secondary  pressure 
at  the  step-down  transformers,  and  the  generator  pressure^ 
to  the  line  voltage.  The  current  values  are,  of  course, 
inversely  changed.  The  power  factor  of  the  load,  hav- 
ing been  assumed  as  .85,  the  induction  factor  will  be 

Vi  -(.85)*  =.52. 


CALCULATION    OF   TRANSMISSION    LINES.      241 


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242       POLYPHASE   APPARATUS   AND    SYSTEMS. 


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CALCULATION   OF   TRANSMISSION   LINES.      243 


In  Chapter  I.,  it  has  been  shown  that  the  impressed 
E.M.F.  is  made  up  of  two  component  parts,  one  in  phase 
with  the  current  and  called  the  energy  component  of  the 
E.M.F.,  the  other  in  quadrature  with  the  current  and 
called  the  induction  component.  In  symbols  : 

Impressed  E.M.F. 


=      S  (Energy  comp.)2  -f  %  (Ind.  coinp.)2 

To  obtain  the  total  E.M.F.,  it  is  necessary,  then,  to  calcu- 
late separately  all  the  energy  and  induction  components  of 
the  circuit,  and  obtain  a  combined  resultant. 

With  the  values  already  assumed,  and  consulting  the 
preceding  table,  we  obtain  the  following  results  : 


CIRCUIT. 

VOLTAGE. 

CURRENT 

AMPERES. 

ENERGY 
COMPO- 
NENT. 

IND.  COM- 
PONENT. 

Secondary  Circuit. 
Energy  Component,       .85  X  5,775, 
Induction  Component,  .52  X  5,775, 
Current, 

Step-down  Transformers. 
Resistance  loss  =  I.R.  =  i%  of  5,775, 
Reactance  loss  =  I.S.  =  3%%  of  5,775, 
Hysteresis  loss  =  i%%  of  102, 

Line. 
Resistance  loss  =  I.R.=  103.5  X  5.72, 
Reactance  loss  =  I.S.  =  103.5  X  6.62, 

4,909 
58 

3,003 
2O2 

I  O2 

4,967 

592 

3,205 
685 

103.5 

J  (5,559)"2  +  (3,890)*  =  6,785=  volts  at 
terminals  of  step-up  transformers. 

Step-up  Transformers. 
Resistance  loss  =  I.R.  =  i%  of  6,785, 
Reactance  loss  =  I.S.  =  3^%  of  6,785, 
Hysteresis  loss                =  i£%of  103.5, 

5,559 
68 

238 

103.5 

>/  (c,  628)2  +  (4,i28V2  —  6,080  —  volts  at 

generator. 

5,627 

4,128 

105. 

244   POLYPHASE  APPARATUS  AND  SYSTEMS. 

The  energy  E.M.F.  between  any  one  line  and  the  neutral 
at  the  generator  end  is  seen  to  be  5,627,  and  the  volts  con- 
sumed by  the  reactance  of  the  system,  4,128.  The  total 
volts  required  at  the  generator  terminals  are  found  to  be 
1 2 1  per  cent  of  the  voltage  at  the  secondaries  of  the  trans- 
formers, reduced  to  the  line  voltage,  —  i.e.,  with  10,000 
equivalent  volts  between  the  lines  at  the  transformer  sec- 
ondaries, the  pressure  at  the  generator  must  be  12,100 
volts.  The  current  delivered  by  the  generator  to  the  line 
is  105  amperes,  and  is  3  per  cent  more  than  the  current  in 
the  secondary  circuits.  The  effect  of  the  transformer  core 
losses  is  the  same  as  if  a  corresponding  current  was  con- 
sumed by  lamps  or  other  apparatus  connected  across  the 
mains.  The  volt-ampere  output  of  the  generator  is  125  per 
cent  of  the  apparent  watts  at  the  receiving  end.  The  power 
factor  of  the  entire  system  is  found  to  be  about  So  per  cent. 

Simple  Wiring  Formulas, —  A  simple  and  sufficiently 
accurate  determination  of  the  sizes  of  conductors,  voltage 
drop,  and  distribution  of  currents,  in  any  direct  or  alternat- 
ing-current system,  can  be  made  from  the  general  formula 
based  on  Ohm's  law,  modified  by  the  use  of  the  proper 
constants.  The  former  formula  and  constants  will  be  found 
especially  useful  and  convenient  for  this  calculation  : 

D  X  W 

Area  of  conductor,  Circular  Mils  =  — —  x  K 

P  X  E 

Volts  loss  in  lines  =  —         -  X  M 

IOO 

W 

Current  in  main  conductors  =  —  x  T 

A_j 

D  =  Distance  of  transmission  (one  way)  in  feet. 
W  =  Total  watts  delivered  to  consumer. 
P  =  Per  cent  loss  in  line  of  W. 

E  =  Voltage  between  main  conductors  at  receiving  or  con- 
sumer's end  of  circuit. 


CALCULATION    OF   TRANSMISSION    LINES.      245 


VALUES  OF  K. 

VALUES  ol  T. 

SYSTEM. 

PER    CENT    POWER    FACTOR. 

PER   CZNT    POWER    FACTOR. 

IOO 

95 

90 

85 

80 

95 

90 

85 

80 

Single-phase  . 

2,160 

2,400 

2,660 

3,OOO 

3,380 

1.052 

I.  Ill 

1.172 

1.250 

Two-phase  (4- 

wire)  .     .     . 

1,  080 

I,2OO 

^33° 

1,500 

1,690 

.526 

•555 

.588 

.625 

Three  -  phase 

(3-wire)   .     . 

1,080 

I,2OO 

1,330 

I,5OO 

1,690 

.607 

.642 

.679 

.725 

Values  of  the  constant,  K,  for  any  particular  power-factor 
are  obtained  by  dividing  2,160  by  the  square  of  that  power 
factor  for  single-phase,  and  by  twice  the  square  of  that 
power  factor  for  three-wire  three-phase  or  four-wire  two- 
phase.  The  resistance  of  line  wire  is  taken  as  10.8  ohms 
per  mil  foot. 

T  is  a  variable,  depending  on  the  system  and  nature  of 
the  load,  and  equal  to  I  for  continuous  current,  and  for 
alternating  current  with  100  per  cent  power-factor.  Its 
value  for  two-phase  and  three-phase  systems  is  .50  and  .58 
respectively,  with  100  per  cent  power-factor. 

M  is  a  variable,  depending  on  the  size  of  wire,  frequency, 
and  power  factor.  It  is  equal  to  I  for  continuous  cur- 
rent, and  for  alternating  current  with  100  per  cent  power- 
factor  and  sizes  of  wire  given  in  the  following  table  of 
wiring  constants. 

The  values  of  M,  as  given  in  the  "table,  are  empirical. 
They  are  sufficiently  accurate  for  all  practical  purposes, 
provided  the  displacement  in  phase  between  current  and 
E.  M.  F.  at  the  receiving  end  is  not  very  much  greater  than 
that  at  the  generator ;  in  other  words,  provided  that  react- 
ance of  the  line  is  not  excessively  large,  or  the  line  loss 
unusually  high.  For  example,  the  constants  should  not  be 


246       POLYPHASE    APPARATUS   AND    SYSTEMS. 


applied  at  125  cycles  if  the  largest-size  conductors  were 
used,  and  the  loss  20  per  cent  or  more  of  the  power  deliv- 
ered. At  lower  frequencies,  however,  the  constants  are 
reasonably  correct,  even  under  such  extreme  conditions. 
They  represent  about  the  true  values  at  10  per  cent  line 
loss,  are  close  enough  at  all  losses  less  than  10  per  cent, 
and  often,  at  least  for  frequencies  up  to  40  cycles,  close 
enough  for  even  much  larger  losses. 

In  using  the  above  formulas  and  constants,  it  should  be 
particularly  observed  that  P  stands  for  the  per  cent  loss  in 
the  line  of  the  delivered  power,  and  not  for  the  per  cent 
loss  in  line  of  the  power  at  the  generator. 


VALUES    OF   M. 

No. 

WEIGHT 
OF  BARE 

30  CYCLES. 

60  CYCLES. 

125  CYCLES. 

OF 

AREA 

WIRE 

WIRE 

CIRCULAR 

PER 

yj 

. 

<«' 

B.  & 

MILS 

o     r  • 

•J     ^   r    '    i     'T                  '        '^ 

•f   ^  '  ' 

r  ' 

c 

r/-      H        •    !     y 

S   G 

1,000   FT. 

2      ;,* 

*  o   •  o  >•   •   ~  >  ""•    -  "  ~ 

~  >  *""• 

—  $• 

g  r  —  .  ,  g  ^.  * 

POUNDS. 

£?-< 

p  "  HH  1  P  —  -^     b"M     h   ~  ^ 

h  ~  "" 

H  Zf-i 

P  "  ^    h  ""  "^ 

|  =^|cf 

>3~! 

ajS 

*°£ 

JC1 

s§s 

scl 

0000 

2Il,6oo 

640.73 

1.26 

1.27 

1.24 

1.64 

1.85 

1.85 

2-44 

3.06 

3.14 

OOO 

167,805 

508.12 

1.20 

1.17 

1.14 

1.49 

1.63 

1.62 

2.15 

2.62 

2.67 

OO 

133,079 

402.97 

I.I5 

1.  08 

1.05 

i-39 

1.46 

1.42 

1.92 

2.25 

2.29 

o 

105,592 

3J9-74 

1.  10 

I.OO 

I.OO 

1.30 

1.32 

1.28 

!-73 

1.96 

1.99 

I 

83,6^4 

253-43 

I.  O6 

I.OO 

I.OO 

1-23 

1.  21 

1.16 

i-57 

1.74 

1-73 

2 

66,373 

200.98 

1.03 

I.OO 

I.OO 

1.16 

I.  II 

i.  06 

1.44 

i-54 

1-53 

3 

52,633 

I59-38 

1.02 

I.OO 

I.OO 

i.  ii 

1.04 

I.OO 

1.35 

1.38 

1.38 

4 

41,742 

126.40 

I.OO 

I.OO 

I.OO 

1.07 

I.OO 

I.OO 

1.26 

1.26 

1.22 

5 

33,102 

100.23 

I.OO 

I.OO 

I.OO 

1.04 

I.OO 

I.OO 

1.19 

1.16 

I.  II 

6 

26,250 

79-49 

I.OO 

I.OO 

I.OO 

i.  02 

I.OO 

I.OO 

1.14 

i.  08 

1.03 

7 

20,816 

63-03 

I.OO 

I.OO 

I.OO 

I.OO 

I.OO 

I.OO 

1.09 

I.OI 

I.OO 

8 

16,509 

49-99 

I.  CO 

I.OO 

I.OO 

I.OO 

I.OO 

I.OO 

i.  06 

I.OO 

I.OO 

CALCULATION    OF   TRANSMISSION    LINES.      247 

APPLICATION  OP  FORMULAS. 
SINGLE-PHASE    SYSTEM. 125    CYCLES. 

EXAMPLE:  750  52-volt  lamps,  consuming  a  total  of 
45,000  watts.  Ratio  of  transformation  20  to  i.  Distance 
to  generator,  2,500  feet.  Loss  in  secondary  wiring,  2  volts. 
Voltage  drops  in  transformers,  2  per  cent.  Energy  loss  in 
line,  5  per  cent  of  delivered  power.  Efficiency  of  trans- 
formers, 97-J  per  cent. 


Watts  at  transformer  primaries 

45,000 


=  47,100. 


.98  x  .97i 
Volts  at  transformer  primaries 

=  (52  +  2)  x  20  X  1.02  =  1,101.6. 

D  X  W       ,>.-     2. coo  X  47,100  X  2.400 

CM.  =  — X  K  =  -^—  -  =  46,500  CM. 

P  X  £-  5  X  (i,ioi.6)a 

Next  larger  B.  &  S.  wire 

=  No.  3  =  52,633  CM. 
Loss  of  delivered  power  using  No.  4  wire 

2, coo  X  47,100  X  2,400 
=  — — z—  Jv2      =  4-4  per  cent. 

52,633  x  (i,ioi.6)2 

Total  volts  lost  in  line 

_  P  x  E  _  4.4  X  1,101.6  X  1.35  _  . 

IOO  IOO 

Generator  voltage        =  1,101.6  -f-  65.5  =  1,167.1. 

In  a  60  cycle  single-phase  system,  with  the  same  condi- 
tions as  in  the  above  example,  the  values  will  be  the  same, 
with  the  exception  of  the  volts  lost  in  the  line. 

4.4  x  1,101.6  x  i. ii  .     ,       .    ,. 

—  =  e-z.8  =  volts  lost  in  line. 

IOO 

1,101.6  +  53.8  =  1,155.4  =  generator  voltage. 


248       POLYPHASE   APPARATUS   AND    SYSTEMS. 

TWO-PHASE  SYSTEM. —  60  CYCLES.     FOUR-WIRE 
TRANSMISSION. 

EXAMPLE  :  2,500  H.P.  delivered,  5  miles,  at  secondaries 
of  step-down  transformers.  Pressure  between  lines  at  re- 
ceiving end,  6,000  volts.  Energy  loss  in  line  and  in  step- 
down  transformers  (no  step-up  transformers),  10  per  cent 
of  delivered  power.  Efficiency  of  transformers,  97.5  per 
cent.  Power  factor  of  load,  80  per  cent.  Find  size  of 
conductors  and  voltage  drop  in  transmission  line. 
Power  delivered  at  step-down  secondaries. 

=  ^^  =  2,564  H.P.  =  1,912.7  K.W. 

Energy  loss  in  line  =  7.5  per  cent. 

„  ...        5,280  X  S  X  1,912,700 

C.M.  —  —  N.t  '       X  1,690  =  3 is, 940  C.M. 

7.5  X  (6,000)" 

Three  No.  o  B.  &  S.  wires  have  an  area  of  316,776  C.M. 
The  energy  loss,  using  3  of  this  size  in  parallel,  making  a 
total  of  1 2  No.  o  B.  &  S.  wires  in  all,  is  : 

5,280  X  5   X  1,912,700 

'       X  1,690  =  7.48  per  cent. 
316,776  x  (6,ooo)2 

Power  lost  in  line 

=  2,564  x  .0748  =  195.8  H.P. 
Volts  lost  in  line 

J>X  E  7.48  x  6,000  X  1.28 

=  -          x  ^/=-  -  =574- 

100  100 

.*.  Generator  voltage  =  6,574. 
Current  in  line 

W  1,012,700 

=  —  x  T—  -  -  X  .625  =  199  amperes. 

Jz  6,000 

The  current  is,  in  fact,  slightly  greater,  as  no  account 
has  been  taken  of  the  hysteresis  current  in  the  trans- 
formers. This  will  increase  the  above  result  about  i^  per 
cent. 


CALCULATION    OF   TRANSMISSION    LINES.      249 

THREE-PHASE    SYSTEM.  -  60    CYCLES.       THREE-WIRE 
TRANSMISSION. 

EXAMPLE  :  Same  conditions  as  preceding.     Find  size  of 
conductors  and  voltage  drop  in  transmission  lines. 

Power  delivered  to  transformers 

=  21^  =  2,564  H.P.  =  1,912.7  K.W. 

Energy  loss  in  line  —  7^  per  cent. 
5,280  X  S  X  1,912,700 

C'M-  =         7.5  x  (6,0007-  "  X  ''  9°  =  3'5>94° 
Three  No.  o  B.  &  S.  wires  have  an  area  of  316,776  C.M. 
For  the  three  branches  of  the  three-phase  system  9  wires 
will  be  required. 

c,28o  x  q  X  1,012,700 

Energy  loss  is  —  —  -—  x  1,600  =  7.48  per  cent. 

316,776  X  (6,000)- 

Power  loss  in  line 

=  2,564  X  .0748  =  195.8  H.P. 
Voltage  drop  in  line 

7.48  x  6,000  X  1.28 


/.  Generator  voltage  =  6,574. 
Current  in  line 

1,012,700 
6,000       X  '72S  =  233' 

The  hysteresis  current  will  increase  this  result  by  about 
\\  per  cent. 

THREE-PHASE    SYSTEM.  -  60    CYCLES.       FOUR-WIRE 
SECONDARY. 

EXAMPLE  :  Required,  the  size  of  conductors  from  trans- 
formers to  the  distributing  centre  of  a  four-wire  secondary 
system  for  lights  and  motors.  The  load  consists  of  four 


250   POLYPHASE  APPARATUS  AND  SYSTEMS. 

1 6  H.P.,  200  volt  induction  motors,  and  750  half-ampere 
1 5  c.p.,  1 1 5  volt  lamps.  Length  of  secondary  wiring  from 
transformers  to  distribution  centre,  600  feet.  About  15 
volts  drop  on  lighting  circuits  from  transformers  to  distrib- 
uting centre.  Efficiency  of  motors,  85  per  cent.  Five 
volts  drop  on  circuits  from  distributing  centre  to  motors. 
Voltage  at  distributing  point  between  main  lines  is  205. 
Current  in  main  lines  for  motors  is 

4  X  i  c  X  746  x  .72^ 

—  =  191  amperes. 

.85  x  200 

Current  for  transformers  from  lamps  is 

(yco  x  -S  X  115)  X  .607 

-^ —  -  =  i  T.I  amperes. 

200 

Total  current  from  transformers  is 

131  -f-  191  —  322  amperes. 
For  motors, 

W  ,,. 

191  =  -  .725.      W  =  54,000. 

For  lamps, 

131  X  -   -  X  .607.      W  =  44.240.      Total  watts  —  98,240. 

Taking  for  trial  two  No.  o  B.  &  S.  wires  in  parallel  for 
each  of  the  main  conductors  as  preferable  to  one  No.  oooo, 

then 

600  X  98,240 

~  2   X   105,592   X  205* 
1,200  X  44,240  +  1,690  X  54,000  _ 

98,240 

Volts  loss  in  lines 

=  9-75  X  205  x  1.32  = 

100 

Volts  at  transformers  between  main  lines  =  231.4. 
Actual  drop  between  main  conductors  and  neutral  to  distrib- 
uting point 

=  26.4  X  —  -  =15.2  volts. 

200 


CALCULATION    OF   TRANSMISSION    LINES.      251 

The  section  of   the  neutral  conductor  should  be  about 

13ix  2x105,592  =  86          CM     We  z   N 

322 

B.  &  S.  wire  with  a  section  of  83,694  C.M.  for  the  neutral. 

MONOCYCLIC     SYSTEM. — -  60    CYCLES.       MOTOR    AND     LIGHTS 
ON    SEPARATE    TRANSFORMERS. 

EXAMPLE:  1,500  half  ampere  104  volt  lamps.  One 
25  H.P.  no  volt  induction  motor;  efficiency,  85  per  cent. 
Distance  from  generator  to  transformer,  3,000  feet.  Dis- 
tance from  transformers  to  motor,  100  feet.  Loss  in  motor 
circuit,  2%  per  cent.  Loss  of  energy  in  transformers,  3  per 
cent.  Loss  in  primary  circuit,  4  per  cent.  Generator 
voltage,  1,040  at  no  load. 

2c  x  746 

Input  at  motor  = =  21,940  watts. 

•°5 

ioo  X  21,940 

C.M.  ==  —  -  X  T>,?>OO  =  241:, ooo  JNo.  oooo 

2.5  X  i  io- 

B.  &  S.  wire  =  211,600   C.M.,  but  as  two   No.  o  B.  &  S.  wires 

will  give  the  same  loss,  and  -    -  —  69.2   per  cent  as  great  a 

1.85 

drop  in  voltage,  they  are  preferable.      Making  each  motor  lead 
of  two  No.  o  B.  &  S.  wires  in  parallel,  then 

ioo  X  21,04.0  X  1,180 

P--  ^      ,    =2.9  per  cent. 

105,592   X  2   X   1 10" 

Volts  loss  to  motors 

2.9  X  no  X  1.28  _ 

ioo 
Volts  at  primaries  of  transformers  for  motors 

=  1.05  X  9  X  (no  -f  4)  =  1,076. 
Volts  on  secondaries  of  lighting  transformers 

1,076 

=  —  —  =  104.  c. 

1.03  X  10 


252   POLYPHASE  APPARATUS  AND  SYSTEMS. 

Watts  at  primaries  of  motor  transformers 

21,040  X  i. 020 
•  =  -  — -  =  23,200. 

•97 
Watts  at  primaries  of  lighting  transformers 

=  ^°°  X'5X  I04'5  =  80,800. 

•97 
Total  watts  delivered  at  transformers 

=  23,200  +  80,800  =  104,000. 
Power  factor  of  load  is 

23,200  x  .80  4-  80,000  x  .95 


104,000 

2,i6o 

A  = x-  =  2,610. 

•91 


=  .91 


_,  3,000  X  104,000 

C.M.  =  -  — —  X  2,610  =  175,500. 

4  X  1,076- 

Taking  No.  ooo  B.  &  S.  wire  X  167,805  C.M.,  then 

3,000  X  104,000 

P=  -  —,  X  2, 6 10  =  4. 19  per  cent. 

167,805   X   1,076" 

Drop  in  primary  circuit 

419  X  1,076        1.49  X  80.8  H-  1.62  X  23.2 

100  104 

=  68.5  volts. 

Voltage  between  outside  lines  at  generator 
=  1,076  +  68.5  =  1,144.5  volts. 
Current  in  main  conductors 

104,000 

=  —  -  =  1 06.  i  amperes. 

1,076  X  .91 

Primary  teaser  wire 

-  x  167,805  =  37,400  C.M.  required. 
104,000 

Use  No.  4  B.  &  S.  wire  with  a  section  of  41,742  C.M. 

Graphical  Illustration The  curves  on  pages  236-239, 

Figs.  167,  1 68,  and  169,  have  been  calculated  from  the 
preceding  formula  and  table  of  constants  relating  to  the 
three-phase  system  only.  They  will  be  found  useful  for 


CALCULATION    OF   TRANSMISSION    LINES.      253 


'S  V9  3H{/MJO  3ZIS 


ool  *  sun  y  vinouM 


254       POLYPHASE    APPARATUS    AND    SYSTEMS. 


CALCULATION    OF   TRANSMISSION    LINES.      255 


'S   V  "ff '3UIM  JO   3ZIS 


8    8    8    1    9    5    § 


s    s 


oooi  x  SHIN 


256   POLYPHASE  APPARATUS  AND  SYSTEMS. 

calculating  and  approximately  determining  the  copper  re- 
quired for  transmitting  any  amount  of  power  any  distance 
at  voltages  varying  from  1,000  to  15,000. 

For  cases  that  fall  outside  the  limits  of  the  curves,  the 
size  of  the  wire  may  be  found  by  applying  the  following 
rules  : 

With  given  power  delivered,  line  loss,  and  voltage,  the  cross- 
section  of  the  conductor  will  vary  directly  as  the  distance. 

With  given  distance  of  transmission,  line  loss,  and  voltage, 
the  cross-section  of  the  conductor  will  vary  directly  as  the  power 
delivered. 

With  given  distance  of  transmission,  power  delivered,  and 
voltage,  the  cross-section  of  the  conductor  will  vary  inversely 
as  the  loss  of  energy  in  the  line. 

With  given  distance,  power  delivered,  and  line  loss,  the  cross- 
section  of  the  conductor  will  vary  inversely  as  the  square  of 
the  voltage. 

The  voltages  are  taken  as  those  at  the  receiving  end. 
The  line  loss  has  been  assumed  to  be  10  per  cent  of  the 
delivered  energy.  In  plotting  the  curves  the  following 
power  factors  have  been  assumed  : 

For  lighting  load 95% 

For  mixed  load  of  induction  motors  and  lights    .      .      85^ 
For  induction  motor  load 80% 

To  illustrate  the  use  of  the  curves,  find  the  size  of  the 
wire  required  to  transmit  5,100  H.P.,  to  be  used  for  incan- 
descent lighting,  a  distance  of  five  miles,  the  current  loss 
being  10  per  cent,  and  the  pressure  at  the  primaries  of 
step-down  transformers,  10,000  volts.  The  curve  (Fig. 
167)  shows  that  each  of  the  three  wires  must  have  a  cross- 
section  of  1 20,000  circular  mils.  If  the  -power  delivered  is 
to  be  consumed  by  induction  motors,  other  conditions  re- 


CALCULATION    OF   TRANSMISSION    LINES.      257 

maining  the  same,  the  conductor  must  have  a  cross-section 
equivalent  to  170,000  circular  mils  each,  or  slightly  larger 
than  No.  ooo  wire.  Or,  supposing  the  wire  to  have  been 
strung  on  the  assumption  that  lights  would  be  supplied, 
the  line  loss  and  pressure  being  the  same  as  above,  it  will 
be  seen  that,  if ,  the  load  is  changed  to  induction  motors, 
only  3,600  H.P.  will  be  delivered  from  these  lines.  This  is  a 
striking  illustration  of  the  decrease  in  the  carrying  capacity 
of  the  line,  due  to  low  power-factors,  which  load  the  line, 
and  the  generators  as  well,  with  so-called  wattless  current. 

If  the  distance  is  raised  to  ten  miles,  the  size  of  wire 
required  for  the  same  transmission  is  doubled  in  both  the 
above  examples.  If  the  distance  is  increased  to  ten  miles, 
and  the  energy  loss  reduced  to  five  per  cent,  the  cross- 
section  of  conductor  will  have  to  be  made  four  times  as 
great. 

Three  wires  of  about  No.  3  size  will  transmit  a  lighting 
load  of  5,100  H.P.  a  distance  of  five  miles,  the  pressure 
being  15,000  at  the  receiving  end.  It  will  take  three  con- 
ductors of  cross-section  corresponding  to  a  size  between 
No.  i  and  No.  2  to  transmit  the  same  power  for  induction 
motor  use,  and  three  No.  2  wires  to  transmit  the  same 
energy  for  a  mixed  load  of  lights  and  motors. 

For  determining  the  size  of  transmission  lines  with  vol- 
tages of  5,000  and  less,  the  curves  in  Fig.  168  will  be 
found  most  convenient. 

Fig.  169  represents  the  curves  of  percentage  drop  of 
voltage  in  transmission  lines,  at  varying  frequencies  and 
power  factors.  The  curves  show  the  values  of  the  con- 
stant, M,  plotted  from  the  table  on  page  238,  and  are  based 
on  10  per  cent  energy  loss  in  line. 

A  study  of   the  curves  shows   some  interesting  facts. 


258       POLYPHASE   APPARATUS    AND    SYSTEMS. 

When  the  transmission  is  effected  at  30  cycles,  it  will  be 
noticed  that,  for  all  commercial  sizes  of  wires,  the  voltage 
drop  is  less  with  a  load  of  low  power-factor  than  with 
one  of  high  power-factor.  For  illustration,  assume  that 
the  transmission  requires  conductors  of  120,000  circular 
mils  each,  the  energy  loss  being  10  per  cent  of  the  deliv- 
ered power.  At  30  cycles,  the  drop  in  voltage  is  10.1 
per  cent  when  the  power  factor  is  80  per  cent,  10.4  per 
cent  when  the  power  factor  is  85  per  cent,  and  11.3  per 
cent  when  the  power  factor  is  95  per  cent.  On  the  other 
hand,  the  same  transmission  at  125  cycles  shows  a  higher 
voltage  drop  with  low  power-factors.  The  voltage  drops 
18.3  per  cent  with  a  95  per  cent  power-factor,  21.2  per 
cent  with  an  85  per  cent  power-factor,  and  21.7  per  cent 
when  the  power  factor  is  80  per  cent. 

A  curious  condition  exists  at  60  cycles.  The  voltage 
drop  is  less  with  a  power  factor  of  95  per  cent,  than  when 
the  power  factor  is  85  per  cent  ;  but  an  80  per  cent 
power-factor  gives  a  drop  approximately  the  same  as  that 
due  to  a  power  factor  of  95  per  cent.  The  curves  also 
graphically  illustrate  the  reduction  in  voltage  drop  to  be 
gained  by  subdividing  the  conductors.  A  No.  oo  wire, 
used  in  a  60  cycle  transmission  of  power  for  induction 
motors,  shows  a  drop  of  14.3  per  cent.  By  subdividing 
the  wire  into  two  No.  2  wires,  and  equivalent  cross-sec- 
tion, the  voltage  drop  is  reduced  to  10.6  per  cent. 

It  will  b?  seen,  from  the  curves,  that,  by  subdividing 
the  conductor  sufficiently,  a  wire  of  a  size  can  be  selected, 
which,  for  all  commercial  power-factors  and  frequencies, 
will  transmit  any  amount  of  power,  with  a  drop  of  voltage 
in  the  line  actually  less  than  the  energy  loss.  This  ap- 
parent anomaly  is  explained  in  Chapter  I.,  under  the  para- 
graph, "Voltage  Drop  Dependent  on  Load  Characteristic." 


CALCULATION    OF   TRANSMISSION   LINES.      259 

Resonance  Effect.  —  What  is  known  as  the  resonance 
effect  of  a  circuit  is  the  rise  of  E.M.F.  at  the  far  end,  above 
that  at  the  generator  end.  This  phenomenon  takes  place 
when  the  natural  period  of  discharge  of  a  circuit  is  equal 
to  the  frequency  of  the  generator  E.M.F.  It  is  complete 
when  the  self-induction  and  capacity  exactly  neutralize 
each  other.  The  charging  current  of  the  line,  due  to  the 
capacity,  then  produces  an  E.M.F.  of  self-induction  equal 
to  the  generator  E.M.F. 

In  transmission  lines,  where  the  inductance  and  capacity 
do  not  exactly  neutralize  each  other,  it  is  possible  for  par- 
tial resonance  to  be  present.  The  circuit  can  be  brought 
into  complete  resonance  by  the  addition  of  a  condenser  or 
a  reactance,  according  as  it  lacks  the  proper  amount  of 
either  capacity  or  inductance.  It  is  conceivable  that  an 
unexpected  rise  of  pressure  may  occur  of  sufficient  extent 
to  destroy  the  insulation  of  line  and  of  apparatus. 

The  rise  of  pressure  due  to  complete  resonance  is  limited 
by  the  ohmic  resistance  of  the  circuit.  For  this  reason, 
and  because  practical  transmissions  of  power  are  accom- 
plished at  a  comparatively  low  frequency,  the  possible  rise 
of  pressure  at  the  receiving  end  is  not  likely  to  be  danger- 
ously high. 

For  very  long  power  transmissions,  where  resonance 
effects  may  be  expected,  it  is  desirable  to  employ  genera- 
tors producing  an  E.M.F.  wave  which  is  sinusoidal.  A 
distorted  wave  of  E.M.F.  of  the  same  period  can  be  resolved 
into  a  number  of  simple  harmonic  components  of  a  higher 
frequency.  These  higher  harmonics  have  the  same  effect 
as  an  E.M.F.  wave  of  the  same  frequency  and  magnitude. 

Another  form  of  resonance  effect  is  that  which  occurs 
during  both  the  making  and  the  disruption  of  a  current  of 


260   POLYPHASE  APPARATUS  AND  SYSTEMS. 

high  tension,  especially  in  long  distance  transmission  lines. 
It  is  found  that  the  oscillatory  discharge  in  the  arc  under 
these  conditions  may  become  of  such  magnitude  as  to  seri- 
ously endanger  the  insulation  of  the  line  and  of  apparatus 
connected  thereto.  As  has  been  described  in  the  section 
under  high  tension  switches,  this  effect  is  particularly 
noticeable  in  those  interruptions  to  the  current  which  are 
effected  in  the  open  air. 


APPENDIX. 

THE  STANDARDIZATION  OF  GENERATORS, 
MOTORS  AND  TRANSFORMERS. 


THE  concentration  of  manufacturing  efforts  along  certain 
lines,  approved  by  experience,  can  be  successfully  carried 
out  by  agreement  between  the  manufacturer  and  the 
consumer  or  his  engineer  acting  as  a  go-between.  The 
tendency  in  the  direction  of  standardization  has  been 
marked  in  the  United  States  during  the  past  few  years. 
It  has  been  made  the  subject  of  formal  action  by  a  number 
of  engineering  associations  working  in  harmony  with  the 
manufacturers.  The  recommendations  and  suggestions 
of  these  bodies  have  been  received  with  approval.  This 
has  been  possible  because  these  recommendations  closely 
conform  to  the  existing  practice  of  the  builders  of  electri- 
cal machinery. 

The  report  of  the  committee  on  standardization  of  the 
American  Institute  of  Electrical  Engineers  is  a  document 
of  great  practical  value  to  electrical  engineers,  and  is  here 
reprinted  in  full. 


262        POLYPHASE    APPARATUS    AND    SYSTEMS. 


REPORT    OF    THE    COMMITTEE    ON 
STANDARDIZATION. 

[Accepted  by  the  INSTITUTE,  June  26th,  1899.] 


To  the  Council  of  The  AMERICAN  INSTITUTE  OF  ELECTRICAL  ENGINEERS. 
Gentlemen  : 

Your  committee  on  Standardization  begs  to  submit  the  following 
report,  covering  such  subjects  as  have  been  deemed  of  pressing  and  imme- 
diate importance,  and  which  are  of  such  a  nature  that  general  agreement 
may  be  expected  upon  them. 

While  it  is  the  opinion  of  the  committee  that  many  other  matters  might 
advantageously  have  been  considered,  as,  for  example,  standard  methods 
of  testing:  yet  it  has  been  deemed  inexpedient  to  attempt  to  cover  in  a 
single  report  more  than  is  here  submitted. 
Yours  respectfully, 

FRANCIS    B.  CROCKER,   Chairman. 

GARY    T.   HUTCHINSON. 

A.  E.   KENNELLY. 

JOHN    W.   LIEB,  JR. 

CHARLES    P.  STEINMETZ. 

LEWIS    B.  STILL  WELL, 

ELIHU    THOMSON. 


GENERAL   PLAN. 

Efficiency.  Sections  i  to  24. 

(I)  Commutating  Machines,  Sections    6  ton 

(II)  Synchronous  Machines,  "  10  to  11 

(III)  Synchronous  Commutating  Machines,  "  12  to  15 

(IV)  Rectifying  Machines,  "  1 6  to  17 

(V)  Stationary  Induction  Apparatus,  "  18  to  19 

(VI)  P.otary  Induction  Apparatus,  "  20  to  23 

(VII)  Transmission  Lines,  "  24 

Rise  of  Temperature.     Sections  25  to  31. 
Insulation.      Sections  32  to  41. 
Regulation.     Sections  42  to  61. 


APPENDIX.  263 

Variation  and  Pulsation.     Sections  62  to  65. 

Rating.     Sections  66  to  73. 

Classification  of  Voltages  and  Frequencies.     Sections  74  to  78. 

Overload  Capacities.     Sections  79  to  82. 

Appendices.     (I)        Efficiency. 

(II)  Apparent  Efficiency. 

(III)  Power  Eactor  and  Inductance  Factor. 

(IV)  Notation. 

(V)  Table  of  Sparking  Distances. 

Electrical  Apparatus  will  be  treated  under  the  following  heads:  — 

I.  Commutating  Machines,  which  comprise  a  constant  magnetic  field, 
a    closed-coil    armature,    and    a    multi-segmental   commutator    connected 
thereto. 

Under  this  head  may  be  classed  the  following:  Direct-current  generators  ; 
direct-current  motors;  direct-current  boosters;  motor-generators;  dyna- 
motors ;  converters  and  closed  coil  arc  machines. 

A  booster  is  a  machine  inserted  in  series  in  a  circuit  to  change  its 
voltage,  and  may  be  driven  either  by  an  electric  motor,  or  otherwise.  In 
the  former  case  it  is  a  motor-booster. 

A  motor-generator  is  a  transforming  device  consisting  of  two  machines  ; 
a  motor  and  a  generator,  mechanically  connected  together. 

A  dynamotor  is  a  transforming  device  combining  both  motor  and  gene- 
rator action  in  one  magnetic  field  with  two  armatures  or  with  an  armature 
having  two  separate  windings. 

For  converters,  see  III. 

II.  Synchronous  Machines,  which  comprise  a  constant  magnetic  field 
and  an  armature  receiving  or  delivering  alternating  currents  in  synchronism 
with  the  motion  of   the  machine  ;  i.e.,  having   a   frequency  equal    to    the 
product  of  the  number  of  pairs  of  poles  and  the  speed  of  the  machine  in 
revolutions  per  second. 

III.  Synchronous  Commutating  Machines :  —These  include:    i.  Syn- 
chronous  converters  ;    i.e.,  converters  from  alternating  to  direct,  or  from 
direct  to  alternating  current,  and  2.   Double  current  generators  ;  i.e.,  gen- 
erators producing  both  direct  and  alternating  currents. 

A  converter  is  a  rotary  device  transforming  electric  energy  from  one  form 
into  another  without  passing  it  through  the  intermediary  form  of  mechanical 
energy. 

A  converter  may  be  either : 

a.  A  direct-current  converter,  converting  from  a  direct  current  to  a 
direct  current,  or 


264   POLYPHASE  APPARATUS  AND  SYSTEMS. 

b.  A  synchronous  converter,  formerly  called  a  rota/y  converter,  convert- 
ing from  an  alternating  to  a  direct  current,  or  vice  versa. 

Phase  converters  are  converters  from  an  alternating-current  system  to  an 
alternating-current  system  of  the  same  frequency  but  different  phase. 

Frequency  converters  are  converters  from  an  alternating-current  system 
of  one  frequency  to  an  alternating-current  system  of  another  frequency 
with  or  without  changes  of  phase. 

IV.  Rectifying  Machines,  or  Pulsating-Current  Generators,  which  pro- 
duce a  unidirectional  current  of  periodically  varying  strength. 

V.  Stationary  Induction  Apparatus,  i.e.,  stationary  apparatus  changing 
electric  energy  from  one  form  into  another,  without  passing  it  through  an 
intermediary  form  of  energy.     These  comprise  : 

a.  Transformers,  or  stationary  induction  apparatus  in  which  the  primary 
and  secondary  windings  are  electrically  insulated  from  each  other. 

b.  Auto-transformers,   formerly  called  compensators  :   i.e.,  stationary  in- 
duction   apparatus    in  which    part    of   the    primary  winding   is  used  as  a 
secondary  winding;   or  conversely. 

c.  Potential  regulators,  or  stationary  induction  apparatus  having  a  coil 
in  shunt,  and  a  coil  in  series  with  the  circuit,  so  arranged  that  the  ratio  of 
transformation  between  them  is  variable  at  will. 

These  may  be  divided  into  :  — 

1.  Compensator   potential-regulators,  in  which  the  number  of   turns  of 
one  of  the  coils  is  changed. 

2.  Induction  potential-regulators,  in  which  the  relative  positions  of  pri- 
mary and  secondary  coils  are  changed. 

3.  Magneto  potential-regulators,  in  which  the  direction  of  the  magnetic 
flux  with  respect  to  the  coils  is  changed. 

d.  Reactive  coils,  or  Reactance  coils,  formerly  called  choking  coils  ;  i.e., 
stationary  induction  apparatus  used  to  produce  impedance  or  phase  dis- 
placement. 

VI.  Rotary  Induction  Apparatus,  which  consist  of  primary  and  secon- 
dary windings  rotating  with  respect  to  each  other.     They  comprise 

a.  Induction  motors. 

b.  Induction  generators. 

c.  Frequency  changers. 

d.  Rotary  phase  converters. 


APPENDIX.  265 


EFFICIENCY. 

1.  The  "efficiency  "  of  an  apparatus  is  the  ratio  of  its  net  power  output 
to  its  gross  power  input.* 

2.  Electric  power  should  be  measured  at  the  terminals  of  the  apparatus. 

3.  In    determining   the    efficiency  of  alternating-current    apparatus,  the 
electric  power  should  be  measured  when  the  current  is  in  phase  with  the 
E.M.F.,  unless  otherwise  specified,  except  when  a  definite  phase 

is  inherent  in  the  apparatus,  as  in  induction  motors,  etc. 

4.  Mechanical  power  in  machines  should    be    measured  at    the  pulley- 
gearing,  coupling,  etc.,  thus  excluding  the  loss  of  power  in  said  pulley,  gear, 
ing  or  coupling,  but  including  the  bearing  friction  and  windage.     The  mag- 
nitude of  bearing  friction  and  windage  may  be  considered  as  independent 
of   the  load.     The  less  of  power  in   the  belt  and  the  increase  of  bearing 
friction    due    to    belt    tension,    should  be    excluded.     Where,    however,  a 
machine  is  mounted    upon  the  shaft  of    a  prime  mover,  in  such  a  manner 
that  it  cannot  be  separated  therefrom,  the  frictional  losses  in  bearings  and 
in  windage,  which  ought,  by  definition,  to   be  included  in  determining  the 
efficiency,  should  be  excluded,  owing  to   the  practical  impossibility  of  de- 
termining them  satisfactorily.       The  brush  friction,    however,    should  be 
included. 

a.  Where  a  machine  has  auxiliary  apparatus,  such  as  an  exciter,  the 
power  lost  in  the  auxiliary  apparatus  should  not  be  charged  to  the  machine 
but  to  the  plant  consisting  of  machine  and  auxiliary  apparatus  taken  to- 
gether. The  plant  efficiency  in  such  cases  should  be  distinguished  from 
the  machine  efficiency. 

5.  The  efficiency  may  be  determined  by  measuring  all  the  losses  individ- 
ually and  adding  their  sum  to  the  output  to  derive  the  input,  or  subtracting 
their   sum  from    the   input    to  derive    the  output.     All   losses    should  be 
measured  at,  or  reduced  to,  the  temperature  assumed  in  continuous  opera- 
tion, or  in  operation  under  conditions  specified.      (See  Sections  25  to  31.) 

In  order  to  consider  the  application  of  the  foregoing  rules  to  various 
machines  in  general  use,  the  latter  may  be  conveniently  divided  into 
classes  as  follows  : 

I.   Commutating  Machines. 

6.  In  commutating  machines  the  losses  are: 

a.    Bearing  friction  and  windage.     (See  Section  4.) 

*  An  exception  should  be  noted  in  the  case  of  storage  batteries  or  apparatus  for  storing 
energy,  in  which  the  efficiency,  unless  otherwise  qualified,  should  be  understood  as  the  ratio 
of  the  energy  output  to  the  energy  intake  in  a  normal  cycle. 


266   POLYPHASE  APPARATUS  AND  SYSTEMS. 

b.  Molecular  magnetic  friction,  and   eddy  currents    in  iron  and  copper. 
These  losses  should   be  determined  with  the  machine   on  open  circuit,  and 
at  a  voltage  equal   to  the  rated  voltage +/r  in  a  generator,  and  — /r  in  a 
motor,  where  /  denotes   the   current    strength,  and  r  denotes  the  internal 
resistance  of  the  machine.     They  should  be  measured  at  the  correct  speed 
and  voltage,  since  they  do  not  usually  vary   in   proportion  to   the  speed  or 
to  any  definite  power  of  the  voltage. 

c.  Armature  resistance   losses,  S2  r' ,  where  7  is   the  current  strength  in 
the  armature,  and  rf  is  the  resistance  between  armature  brushes,  excluding 
the  resistance  of  brushes  and  brush  contacts. 

d.  Commutator  brush  friction, 

e.  Commutator   brush-contact  resistance.     It  is   desirable  to  point  out 
that  with  carbon  brushes  the  losses  (d)  and  (<?)  are  usually  considerable  in 
low-voltage  machines. 

/  Field  excitation.  With  separately  excited  fields,  the  loss  of  power  in 
the  resistance  of  the  field  coils  alone  should  be  considered.  With  shunt 
fields  or  series  fields,  however,  the  loss  of  power  in  the  accompanying  rheo- 
stat should  also  be  included,  the  said  rheostat  being  considered  as  an  essen- 
tial part  of  the  machine,  and  not  as  separate  auxiliary  apparatus. 

(b)  and  (c)  are  losses  in  the  armature  or  "  armature  losses  ;  "  (d)  and  (e) 
"commutator  losses  ;  "  (/)  "  field  losses." 

7.  The  difference  between  the  total  losses  under  load  and  the  sum  of 
the  losses  above  specified,  should  be   considered  as  "  load  losses,"  and  are 
usually  trivial  in  commutating  machines  of  small  field  distortion.    When  the 
field    distortion    is    large,    as    is  shown    by  the  necessity    for    shifting   the 
brushes  between  no   load  and  full   load,  or  with  variations   of  load,  these 
load  losses  may  be  considerable,  and  should  be  taken   into  account.     In 
this  case  the  efficiency  may  be  determined  either  by  input  and  output  meas- 
urements, or  the  load-losses  may  be  estimated  by  the  method  of  Section  II. 

8.  Boosters   should   be  considered  and  treated  like  other  direct-current 
machines  in  regard  to  losses. 

9.  In   motor-generators,  dynamotors  or  converters,  the   efficiency  is  the 
electric  output 

electric  input. 

II.   Synchronous  Machines. 

10.  In  synchronous  machines  the  output  or  input  should  be  measured 
with  the  current  in  phase  with  the  terminal  £.  M,  F.  except  when  otherwise 
expressly  specified. 

Owing  to  the  uncertainty  necessarily  involved  in  the  approximation  of 
load  losses,  it  is  preferable,  whenever  possible,  to  determine  the  efficiency 
of  synchronous  machines  by  input  and  output  tests. 


APPENDIX.  267 

11.  The  losses  in  synchronous  machines  are : 

a.  Bearing  friction  and  windage  ;  see  Sec.  4. 

b.  Molecular  magnetic  friction   and   eddy  currents  in  iron,  copper  and 
other  metallic  parts.     These  losses  should  be  determined  at  open  circuit  of 
the  machine  at  the  rated  speed   and  at   the  rated  voltage.      +  Ir  in  a  syn- 
chronous generator, — Ir  in  a  synchronous  motor,   where    /—current  in 
armature,  r  =  armature  resistance.      It   is  undesirable  to  compute  these 
losses  from  observations  made  at  other  speeds  or  voltages. 

These  losses  may  be  determined  either  by  driving  the  machine  by  a  mo. 
tor,  or  by  running  it  as  a  synchronous  motor,  and  adjusting  its  fields  so  as 
to  get  minimum  current  input  and  measuring  the  input  by  wattmeter.  The 
former  is  the  preferable  method,  and  in  polyphase  machines  the  latter 
method  is  liable  to  give  erroneous  results  in  consequence  of  unequal  distri- 
bution of  currents  in  the  different  circuits  caused  by  inequalities  of  the 
impedance  of  connecting  leads,  etc. 

c.  Armature-resistance  loss,  which  may  be  expressed  by/  72  r ;  where  r 
•=  resistance  of  one  armature  circuit  or  branch,./ =  the  current  in  such  arma- 
ture circuit  or  branch,  and  /  —  the  number  of  armature  circuits  or  branches. 

d.  Load  losses  as  denned  in  section  7.     While  these  losses  cannot  well 
be  determined  individually,  they  may  be  considerable,  and,  therefore,  their 
joint  influence  should  be  determined  by  observation.     This  can  be  done  by 
operating  the  machine  on  short  circuit  and   at  full-load  current,  that  is,  by 
determining  what  may  be  called  the  "  short-circuit   core  loss."     With  the 
low  field  intensity  and  great  lag  of  current  existing  in  this   case,  the  load 
losses  are  usually  greatly  exaggerated. 

One  third  of  the  short-circuit  core  loss  may,  as  an  approximation,  and  in 
the  absence  of  more  accurate  information,  be  assumed  as  the  load  loss. 

e.  Collector-ring  friction   and   contact  resistance.     These   are   generally 
negligible,  except  in  machines  of  extremely  low  voltage. 

f.  Field   excitation.      In  separately  excited   machines,  the   I'2  r  of  the 
field  coils  proper  should  be  used.     In  self-exciting  machines,  however,  the 
loss  in  the  field  rheostat  should  be  included,     (See  Section  6f.) 

III.   Synchronous  Commutating  Machines. 

12.  In  synchronous  converters,  the  power  on  the  alternating-current  side 
is  to  be  measured  with  the   current    in   phase  with   the  terminal  E.M.Fs 
unless  otherwise  specified. 

13.  In  double-current  generators,  the  efficiency  of  the  machine  should  be 
determined  as  a  direct-current  generator  in  accordance  with  section  6,  and 
as  an  alternating  current   generator  in   accordance  with   section    n.     The 
two    values    of   efficiency   may    be  different,    and    should   be    clearly    dis- 
tinguished. 


268        POLYPHASE    APPARATUS   AND    SYSTEM. 

14.  In  synchronous  converters  the  losses   should  be  determined  when 
driving  the  machine  by  a  motor.     These  losses  are  :  — 

a.  Bearing  friction  and  windage,  see  section  4. 

b.  Molecular  magnetic  friction  and  eddy  currents  in  iron,  copper  and  me- 
tallic parts.     These  losses  should  be  determined  at  open  circuit  and  at  the 
rated  terminal  voltage,  no  allowance  being  made  for  the  armature   resis- 
tance,   since    the    alternating    and    the    direct    currents    flow   in    opposite 
directions. 

c.  Armature  resistance.     The  loss  in  the   armature  is  q  I'2  ;-,  where  /— 
direct  current   in  armature,  r  =  armature   resistance  and  q,  a  factor  which 
is  equal  to    1.37  in   single-phasers,  0.56   in   three-phasers,  0.37   in  quarter- 
phasers  and  0.26  in  six-phasers. 

d.  Load  losses.     The  load  losses   should  be  determined  in  the  same 
manner  as  described  in    section  1 1  </,  with   reference   to  the  direct-current 
side. 

e  and/!  Losses  in  commutator  and  collector  friction  and  brush-contact 
resistance.  See  sections  6  and  11. 

g.  Field  excitation.  In  separately-excited  fields,  the  72  r  loss  in  the  field 
coils  proper  should  be  taken,  while  in  shunt  and  series  fields  the  rheostat 
loss»should  be  included,  except  where  fields  and  rheostats  are  intentionally 
modified  to  produce  effects  outside  of  the  conversion  of  electric  power,  as 
for  producing  phase  displacement  for  voltage  control.  In  this  case  25  per 
cent  of  the  /2r  loss  in  the  field  proper  at  non-inductive  alternating  circuit 
should  be  added  as  proper  estimated  allowance  for  normal  rheostat  losses. 
(See  Section  6f.) 

15.  Where  two   similar  synchronous   machines  are  available,  their  effi- 
ciency can  be   determined  by  operating  one  machine  as  a  converter  from 
direct  to  alternating,  and  the  other  as  a  converter  from  alternating  to  direct, 
connecting  the  alternating  sides  together,  and  measuring  the  difference  be- 
tween the  direct-current   input,  and  the   direct-current   output.     This  pro- 
cess may  be    modified   by  returning  the   output    of   the   second    machine 
through   two  boosters   into   the  first    machine   and   measuring  the  losses. 
Another  modification   might  be  to   supply  the   losses   by  an   alternator  be- 
twesn  the  two  machines,  using  potential  regulators. 

IV.   Rectifying  Machines  or  Pulsating-Current  Generators. 

16.  These  include  :     Open-coil  arc  machines,  constant-current  rectifiers, 
constant-potential  rectifiers. 

The  losses  in  open-coil  arc  machines  are  essentially  the  same  as  in  sec- 
tions 6  to  9  (closed-coil  commutating  machines).  In  alternating-current 
rectifiers,  however,  the  output  must  be  measured  by  wattmeter  and  not  by 
voltmeter  and  ammeter,  since  owing  to  the  pulsation  of  current  and  E.M.F., 


APPENDIX.  269 

a  considerable    discrepancy  may    exist    between  watts    and  volt-amperes, 
amounting  to  as  much  as  10  or  15  per  cent. 

17.  In  constant  current  rectifiers,  transforming  from  constant-potential 
alternating  to  constant  direct  current  by  means  of  constant-current   trans- 
formers and  rectifying  commutators,  the  losses  in  the  transformers  are  to  be 
included  in  the   efficiency  and  have  to  be   measured  when,   operating  the 
rectifier,  since  in  this  case  the  losses  are  generally  greater  than  when  feed- 
ing an  alternating  secondary  circuit.     In  constant -current   transformers  the 
load  losses  are  usually  larger  than  in  constant-potential   transformers  and 
thus  should  not  be  neglected. 

The  most  satisfactory  method  of  determining  the  efficiency  in  rectifiers  is 
to  measure  electric  input  and  electric  output  by  wattmeter.  The  input  is 
usually  not  non-inductive,  owing  to  a  considerable  phase  displacement  and 
to  wave  distortion.  For  this  reason  the  apparent  efficiency  should  also  be 
considered,  since  it  is  usually  much  lower  than  the  true  efficiency.  The 
power  consumed  by  the  synchronous  motor  or  other  source  driving  the 
rectifier  should  be  included  in  the  electric  input. 

V.   Stationary  Induction  Apparatus. 

18.  Since  the  efficiency  of  induction  apparatus  depends  upon  the  wave 
shape  of  E,M.F\,  it  should  be  referred  to  a  sine  wave  of  E.M.F.,  except 
where  expressly  specified   otherwise.     The   efficiency  should  be  measured 
with  non-inductive  load,  and   at  rated  frequency,  except  where   expressly 
specified  otherwise.     The  losses  are : 

a.  Molecular  magnetic  friction  and.  eddy  currents   measured  at  open  cir- 
cuit and  at  rated  voltage  —  Ir,  where  I—  rated  current  r=  resistance  of 
primary  circuit. 

b.  Resistance  losses,  the  sum  of  the  I2  r  of   primary  and   of  secondary 
in  a  transformer,  or  of  the  two  sections  of  the  coil  in  the  compensator  or 
auto-transformer,  where  /=  current  in  the  coil  or  section  of  coil,  r  —  resist- 
ance. 

c.  Load  losses,  i.e.,  eddy  currents  in  the  iron  and  especially  in  the  cop- 
per conductors,  caused  by  the  current.     They  should  be  measured  by  short- 
circuiting  the  secondary  of  the  transformer  and  impressing  upon  the  primary 
an  E.M.F.    sufficient  to   send   full-load    current  through  the  transformer. 
The  loss  in  the  transformer  under  these  conditions  measured  by  wattmeter 
gives  the  load  losses +  /2  r  losses  in  both  primary  and  secondary  coils. 

d.  Losses  due  to  the  methods   of   cooling,  as  power  consumed  by  the 
blower  in  air-blast  transformers,  and  power  consumed  by  the  motor  driving 
pumps  in  oil   or  water    cooled    transformers.     Where    the    same    cooling 
apparatus  supplies  a  number  of  transformers  or  is  installed  to  supply  future 
additions,  allowance  should  be  made  therefor. 


2/0   POLYPHASE  APPARATUS  AND  SYSTEMS. 

19.  In  potential  regulators  the   efficiency  should  be   taken  at  the  maxi- 
mum voltage  for  which   the  apparatus  is  designed,  and  with  non-inductive 
load,  unless  otherwise  specified. 

VI.   Rotary  Induction  Apparatus. 

20.  Owing  to  the  existence  of  load  losses,  and  since  the  magnetic  density 
in  the  induction  motor  under  load  changes   in   a  complex  manner,  the  effi- 
ciency should  be  determined  by  measuring  the  electric  input   by  wrattmeter 
and  the  mechanical  output  at  the  pulley,  gear,  coupling,  etc. 

21.  The  efficiency  should  be  determined  at  the  rate  of  frequency  and  the 
input  measured  with  sine  waves  of  impressed  E.AI.F. 

22.  The  efficiency  may  be  calculated  from  the  apparent  input,  the  power 
factor,  and  the  power  output.     The  same  applies  to  induction  generators, 
Since  phase  displacement  is  inherent  in  induction  machines,  their  appar- 
ent efficiency  is  also  important. 

23.  In   frequency  changers  ;    i.e.,  apparatus   transforming  from  a  poly- 
phase system  to  an  alternating  system  of  different  frequency,  with  or  with- 
out a  change  in  the  number  of  phases,  and  phase  converters;  i.e.,  apparatus 
converting  from   an   alternating   system,  usually  single  phase,  to  another 
alternating  system,  usually  polyphase,  of  the  same  frequency,  the  efficiency 
should  also  be  determined  by  measuring  both  output  and  input. 

VII.     Transmission  Lines. 

24.  The  efficiency  of  transmission  lines  should  be  measured  with  non- 
inductive  load  at  the  receiving  end  with  the  rated  receiving  pressure  and 
frequency,  also  with  sinusoidal  impressed  E.M.FSs.,  except  where  expressly 
specified  otherwise,  and  with  the   exclusion  of  transformers  or  other  ap- 
paratus at  the  ends  of  the  line. 


RISE    OF   TEMPERATURE. 

General  Principles. 

25.  Under  regular  service  conditions,  the  temperature  of  electrical  ma- 
chinery should  never  be  allowed  to  remain  at  a  point  at  which  permanent 
deterioration  of  its  insulating  material  takes  place. 

26.  The  rise  of  temperature  should  be  referred  to  the  standard  conditions 
of  a  room-temperature  of  25°  C.,  a  barometric  pressure  of  760  mm.  and 
normal  conditions  of  ventilation ;  that  is,  the  apparatus  under  test  should 
neither  be  exposed  to  draught  nor  enclosed,  except  where  expressly  specified. 

27.  If  the    room-temperature    during  the  test   differs  from  25°  C.,  the 
observed  rise  of  temperature  should  be  corrected  by  \  per  cent  for  each 


APPENDIX. 


271 


degree  C.*  Thus  with  a  room-temperature  of  35°  C.,  the  observed  rise  of 
temperature  has  to  be  decreased  by  5  per  cent,  and  with  a  room-tempera- 
ture of  15°  C.,  the  observed  rise  of  temperature  has  to  be  increased  by  5 
per  cent.  The  thermometer  indicating  the  room-temperature  should  be 
screened  from  thermal  radiation  emitted  by  heated  bodies,  or  from  draughts 
of  air.  When  it  is  impracticable  to  secure  normal  conditions  of  ventilation 
on  account  of  an  adjacent  engine,  or  other  sources  of  heat,  the  thermometer 
for  measuring  the  air  temperature  should  be  placed  so  as  fairly  to  indi- 
cate the  temperature  which  the  machine  would  have  if  it  were  idle,  in  order 
that  the  rise  of  temperature  determined  shall  be  that  caused  by  the  opera- 
tion of  the  machine. 

28.  The  temperature  should  be  measured  after  a  run  of  sufficient  dura- 
tion to  reach   practical  constancy.     This   is  usually  from  6  to   18   hours, 
according  to  the  size  and  construction  of  the  apparatus.     It  is  permissible, 
however,  to  shorten  the  time  of  the  test  by  running  a  lesser  time  on  an 
overload  in  current  and  voltage,  then  reducing  the  load  to  normal,  and 
maintaining  it  thus  until  the  temperature  has  become  constant. 

In  apparatus  intended  for  intermittent  service,  as  railway  motors,  starting 
rheostats,  etc.,  the  rise  of  temperature  should  be  measured  after  a  shorter 
time,  depending  upon  the  nature  of  the  service,  and  should  be  specified. 

In  apparatus  which  by  the  nature  of  their  service  may  be  exposed  to 
overload,  as  railway  converters,  and  in  very  high  voltage  circuits,  a  smaller 
rise  of  temperature  should  be  specified  than  in  apparatus  not  liable  to 
overloads  or  in  low  voltage  apparatus.  In  apparatus  built  for  conditions  of 
limited  space,  as  railway  motors,  a  higher  rise  of  temperature  must  be  allowed. 

29.  In  electrical  conductors,  the  rise  of  temperature  should  be  deter- 
mined by  their  increase  of  resistance.     For  this  purpose  the  resistance  may 
be  measured  either  by  galvanometer  test,  or  by  drop-of-potential  method. 
A  temperature  coefficient  of  0.4  per  cent,  per  degree   C.  may  be  assumed 
for  copper.t     Temperature  elevations  measured  in  this  way  are  usually  in 
excess  of  temperature  elevations  measured  by  thermometers. 

30.  It  is  recommended  that  the  following  maximum  values  of  tempera- 
ture elevation  should  not  be  exceeded : 

Commutating  machines,  rectifying  machines,  and  synchronous  machines. 
Field  and  armature,  by  resistance,  50°  C. 

Commutator  and  collector  rings  and  brushes,  by  thermometer,  55°  C. 
Bearings  and  other  parts  of  machine,  by  thermometer,  40°  C. 

*  This  correction  is  also  intended  to  compensate,  as  nearly  as  is  at  present  practicable, 
for  the  error  involved  in  the  assumption  of  a  constant  temperature  coefficient  of  resistivity  ; 
i.e.,  0.4  per  cent  per  deg.  C.  taken  with  varying  initial  temperatures. 

t  By  the  formula  ^?T=r  ^?*(i  -}-o.oo.|-0).  Where  Rt  is  the  resistance  at  room-temperature, 
RT:  the  resistance  when  heated,  and  &  the  temperature  elevation  (T — t]  in  degrees  centigrade. 


272       POLYPHASE   APPARATUS   AND    SYSTEMS. 

Rotary  induction  apparatus  : 

Electric  circuits,  50°  C.,  by  resistance. 

Bearings  and  other  parts  of  the  machine  40°  C.,  by  thermometer. 

In  squirrel-cage  or  short-circuited  armatures,  55°  C.,  by  thermometer, 
may  be  allowed. 

Transformers  for  continuous  service  —  electric  circuits  by  resistance 
50°  C.,  other  parts  by  thermometer,  40°  C.,  under  conditions  of  normal 
ventilation. 

Reactive  coils,  induction  and  magnetic  regulators  —  electric  circuits  by 
resistance  55°  C.,  other  parts  by  thermometer  45°  C. 

Where  a  thermometer,  applied  to  a  coil  or  winding,  indicates  a  higher 
temperature  elevation  than  that  shown  by  resistance  measurement,  the 
thermometer  indication  should  be  accepted.  In  using  the  thermometer, 
care  should  be  taken  so  to  protect  its  bulb  as  to  prevent  radiation  from  it, 
and,  at  the  same  time,  not  to  interfere  seriously  with  the  normal  radiation 
from  the  part  to  which  it  is  applied. 

31.  In  the  case  of  apparatus  intended  for  intermittent  service,  the  tem- 
perature elevation  which  is  attained  at  the  end  of  the  period  corresponding 
to  the  term  of  full  load,  should  not  exceed  50°  C.  by  resistance  in  electric 
circuits.     In  the  case  of  transformers  intended  for  intermittent  service,  or 
not  operating  continuously  at  full  load,  but  continuously  in  circuit,  as  in 
the  ordinary  case  of  lighting  transformers,  the  temperature  elevation  above 
the  surrounding   air-temperature   should   not    exceed  50°  C.  by  resistance 
in   electric   circuits   and  40°  C.  by  thermometer  in  other  parts,  after  the 
period  corresponding  to  the  term  of  full  load.     In  this  instance,  the  test 
load  should  not  be  applied  until  the  transformer  has  been  in  circuit  for  a 
sufficient  time  to  attain  the  temperature  elevation  due  to  core  loss.     With 
transformers    for  commercial    lighting,  .the   duration   of  the  full-load  test 
may  be  taken  as  three  hours,  unless  otherwise  specified.     In  the  case  of 
railway,  crane  and  elevator  motors,  the  conditions  of  service  are   neces- 
sarily so  varied  that  no  specific  period  corresponding  to  the  full-load  term 
can  be  stated. 

INSULATION. 

32.  The  ohmic  resistance  of  the  insulation  is  of  secondary  importance 
only,  as  compared  with  the  dielectric  strength,  or  resistance  to  rupture  by 
high  voltage. 

Since  the  ohmic  resistance  of  the  insulation  can  be  very  greatly  increased 
by  baking,  but  the  dielectric  strength  is  liable  to  be  weakened  thereby,  it 
is  preferable  to  specify  a  high  dielectric  strength  rather  than  a  high  insula- 


APPENDIX.  273 

lion  resistance.  The  high  voltage  test  for  dielectric  strength  should  always 
be  applied. 

Insulation  Resistance. 

33.  Insulation  resistance  tests  should,  if  possible,  be  made  at  the  pres. 
sure  for  which  the  apparatus  is  designed. 

The  insulation  resistance  of  the  complete  apparatus  must  be  such  that 

the  rated  voltage  of  the  apparatus  will  not  send  more  than  of  the 

i  ,000,000 

full-load  current,  at  the  rated  terminal  voltage,  through  the  insulation. 
Where  the  value  found  in  this  way  exceeds  i  megohm,  i  megohm  is 
sufficient. 

Dielectric  Strength. 

34.  The  dielectric  strength  or  resistance  to  rupture  should  be  determined 
by  a  continued  application  of  an  alternating  E.M.F.  for  one  minute.      The 
source  of  alternating  E.M.F.  should  be  a  transformer  of  such  size  that  the 
changing  current  of  the  apparatus  as  a  condenser   does  not  exceed  25%  of 
the  rated  capacity  of  the  transformer. 

35.  The  high  voltage  tests  should  not  be  applied  when  the  insulation  is 
low,  owing  to  dirt  or  moisture,  and  should  be  applied  before  the  machine 
is  put  into  commercial  service. 

36.  It  should  be  pointed  out  that  tests  at  high  voltages  considerably  in 
excess  of  the  normal  voltages  are  admissible  on  new  machines,  to  determine 
whether  they  fulfill  their  specifications,  but  should    not    be    made   subse- 
quently at  a  voltage  much  exceeding  the  normal,  as  the  actual  insulation 
of  the  machine  may  be  weakened  by  such  tests. 

37.  The  test  for  dielectric  strength  should  be  made  with  the  completely 
assembled   apparatus   and   not   with  its  individual  parts,  and   the    voltage 
should  be  applied  as  follows  :  — 

i st.    Between  electric  circuits  and  surrounding  conducting  material,  and, 

2d.  Between  adjacent  electric  circuits,  where  such  exist,  as  in  trans- 
formers. 

The  tests  should  be  made  with  a  sine  wave  of  E.M.F.,  or  where  this  is 
not  available,  at  a  voltage  giving  the  same  striking  distance  between  needle 
points  in  air,  as  a  sine  wave  of  the  specified  E.M.F.,  except  where  ex- 
pressly specified  otherwise.  As  needles,  new  sewing  needles  should  be 
used.  It  is  recommended  to  shunt  the  apparatus  during  the  test  by  a 
spark  gap  of  needle  points  set  for  a  voltage  exceeding  the  required  voltage 
by  10%. 

A  table  of  approximate  sparking  distances  is  given  in  Appendix  V. 


2/4   POLYPHASE  APPARATUS  AND  SYSTEMS. 

38.    The  following  voltages  are  recommended  for  apparatus  not  including 
transmissions  line  or  switchboards  : 


RATED  TERMINAL  VOLTAGE. 

CAPACITY.          TESTING  VOLTAGE. 

Not  exceeding 

400  volts      .... 

Under  10  K.YV.    . 

1000  volts. 

" 

10  K.W.  and  over, 

1500      " 

400  and  over,  but  less  than  800  volts  . 

Under  10  K.W.    . 

1500      " 

"          " 

"             "       " 

10  K.W.  and  over, 

2000        " 

Soo 

"                 I2OO 

Any     

3500    » 

200            " 

"                2500 

Any     

5000     '• 

1500           " 

"                     .       .       .        . 

Any     .     .     .      .    j 

Double  the  normal 
rated  voltaees. 

Synchronous  motor  fields  and  fields  of  converters  started 

from  the  alternating  current  side 5000  volts. 

Alternator  field  circuits  should  be  tested  under  a  breakdown  test  voltage 
corresponding  to  the  rated  voltage  of  the  exciter,  and  referred  to  an  output 
equal  to  the  output  of  the  alternator;  i.  e.,  the  exciter  should  be  rated  for 
this  test  as  having  an  output  equal  to  that  of  the  machine  it  excites. 

Condensers  should  be  tested  at  twice  their  rated  voltage  and  at  their 
rated  frequency. 

The  values  in  the  table  above  are  effective  values,  or  square  roots  of 
mean  square  reduced  to  a  sine  wave  of  E.M.F. 

39.  In  testing  insulation  between  different  electric  circuits,  as  between 
primary  and  secondary  of  transformers,  the  testing  voltage  must  be  chosen 
corresponding  to  the  high-voltage  circuit. 

40.  In  transformers  of  from   10,000  volts  to  20,000  volts,  it  should  be 
considered  as  sufficient  to  operate  the  transformer  at   twice  its  rated  vol- 
tage, by  connecting  first  the  one,  and  then  the  other  terminal  of  the  high- 
voltage  winding  to  the  core  and  to  the  low-voltage  winding.     The  test  of 
dielectric  resistance  between  the  low-voltage  winding  and  the  core  should 
be    in    accordance    with    the    recommendation    in    Section  38,   for    similar 
voltages  and  capacities. 

41.  When  machines  or  apparatus  are  to  be  operated  in  series,  so  as  to 
employ  the  sum  of  their  separate  £.JS.F.'s  the  voltage  should  be  referred 
to  this  sum,  except   where  the  frames  of  the  machines  are  separately  in- 
sulated both  from  ground  and  from  each  other. 

REGULATION. 

42.  The  term  regulation  should  have  the  same  meaning  as  the  term  '•  in- 
herent regulation,"  at  present  frequently  used. 

43.  The  regulation  of  an  apparatus  intended  for  the  generation  of  con- 


APPENDIX.  2/5 

stant  potential,  constant  current,  constant  speed,  etc.,  is  to  be  measured 
by  the  maximum  variation  of  potential,  current,  speed,  etc.,  occurring 
within  the  range  from  full  load  to  no  load,  under  such  constant  con- 
ditions of  operation  as  give  the  required  full-load  values,  the  condition  of 
full  load  being  considered  in  all  cases  as  the  normal  condition  of  opera- 
tion. 

44.  The  regulation  of  an  apparatus  intended  for  the  generation  of   a 
potential,  current,  speed,  etc.,   varying  in   a  definite  manner   between  full 
load  and  no  load,  is  to  be  measured  by  the  maximum  variation  of  potential, 
current,  speed,  etc.,  from  the  satisfied  condition,  under  such  constant  con- 
ditions of  operation  as  give  the  required  full-load  values. 

If  the  manner  in  wrhich  the  variation  in  potential,  current,  speed,  etc., 
between  full  load  and  no  load  is  not  specified,  it  should  be  assumed  to  be 
a  simple  linear  relation  ;  i.  e.,  undergoing  uniform  variation  between  full 
load  and  no  load. 

The  regulation  of  an  apparatus  may,  therefore,  differ  according  to  its 
qualification  for  use.  Thus  the  regulation  of  a  compound-wound  generator 
specified  as  a  constant-potential  generator  will  be  different  from  that  it 
possesses  when  specified  as  an  over-compounded  generator. 

45.  The  regulation  is  given  in   percentage  of  the  full-load    value  of  po- 
tential, current,  speed,  etc.,  and  the  apparatus  should  be  steadily  operated 
during  the  test  under  the  same  conditions  as  at  full  load. 

46.  The  regulation  of  generators  is  to  be  determined  at  constant  speed  ; 
of  alternating  apparatus  at  constant  impressed  frequency. 

47.  The  regulation  of  a  generator-unit,  consisting  of  a  generator  united 
with  a  prime-mover,  shoxild  be   determined  at  constant  conditions  of  the 
prime  mover;   i.  e.,  constant  steam  pressure,  head,  etc.     It  would  include 
the  inherent  speed  variations   of  the   prime-mover.     For    this  reason  the 
regulation  of  a  generator-unit  is  to  be  distinguished  from  the  regulation  of 
either  the  prime-mover,  or  of  the  generator  contained  in  it,  when  taken 
separately. 

48.  In  apparatus    generating,  transforming   or   transmitting    alternating 
currents,  regulation  should  be  understood   to  refer  to  non-inductive  load, 
that   is    to    a  load  in    which  the  current  is  in  phase  with  the  E.1W.F.  at 
the  output  side  of  the  apparatus,  except  wrhere  expressly  specified  other- 
wise. 

49.  In  alternating  apparatus  receiving  electric  power,  regulation  should 
refer  to  a  sine  wave  of  E.M.F.,  except  \vhere    expressly  specified    other- 
wise. 

50.  In  commutating  machines,  rectifying  machines  and  synchronous  ma- 
chines, as    direct -current   generators  and  motors,   alternating-current    and 


2/6   POLYPHASE  APPARATUS  AND  SYSTEMS. 

polyphase  generators,  the  regulation  is  to  be  determined  under  the  following 
conditions  : 

a.  At  constant  excitation  in  separately  excited  fields. 

b.  With  constant  resistance  in  shunt  fields  circuits,  and 

c.  With  constant  resistance  shunting  series  fields;  i.e.,  the  field  adjust- 
ment should  remain   constant,  and    should  be   so   chosen  as    to  give    the 
required  full-load  voltage  at  full-load  current. 

51.  In  constant    potential   machines,  the    regulation  is    the  ratio  of  the 
maximum   difference    of  terminal    voltage   from   the  rated   full-load  value 
(occurring  within  the  range  from  the  full  load  to  open  circuit)  to  the  full- 
load  terminal  voltage. 

52.  In  constant-current  machines,  the  regulation  is  the  ratio  of  the  maxi- 
mum difference  of  current  from  the  rated  full-load  value  (occurring  within 
the  range  from  full-load  to  short-circuit),  to  the  full-load  current. 

53.  In  constant  power  machines,  the  regulation  is  the  ratio  of  maximum 
difference  of  power  from   the  rated  full-load    value  (occurring  within  the 
range  of  operation  specified)  to  the  rated  power. 

54.  In    over-compounded  machines,  the    regulation  is   the  ratio  of  the 
maximum  difference  in  voltage  from  a  straight  line  connecting  the  no-load 
and  full-load   values  of  terminal  voltage  as  function  of  the  current,  to  the 
full-load  terminal  voltage. 

55.  In  constant-speed   continuous-current  motors,  the    regulation  is  the 
ratio  of  the  maximum  variation  of  speed  from  its  full-load  value  (occurring 
within  the  range  from  full-load  to  no-load)  to  the  full-load  speed. 

56.  In  transformers,  the  regulation  is  the  ratio  of  the   rise  of  secondary 
terminal   voltage  from  full-load  to  no-load  (at  constant  primary  impressed 
terminal  voltage)  to  the  secondary  terminal  voltage. 

57.  In  induction  motors,  the  regulation  is  the  ratio  of  the  rise  of  speed 
from  full-load  to   no-load  (at  constant  impressed  voltage),  to  the  full-load 
speed. 

The  regulation  of  an  induction  motor  is,  therefore,  not  identical  with  the 
slip  of  the  motor,  which  is  the  ratio  of  the  drop  in  speed  from  synchronism, 
to  the  synchronous  speed. 

58.  In  converters,    dynamotors,   motor  generators,  and  frequency  chan- 
gers, the  regulation  is  the  ratio  of    the  maximum  difference  of   terminal 
voltage  at  the    output  side  from   the   rated  full-load  voltage  (at  constant 
impressed  voltage  and  at  constant   frequency)  to  the  full-load  voltage  on 
the  output  side. 

59.  In   transmission  lines,  feeders,  etc..   the  regulation   is   the  ratio  of 
maximum  voltage  difference   at    the    receiving  end,  between  no-load   and 
full   non-inductive  load,  to  the  full-load  voltage  at  the  receiving  end,  with 
constant  voltage  impressed  upon  the  sending  end. 


APPENDIX.  277 

60.  In    steam  engines,  the  regulation  is  the   ratio   of  maximum  varia- 
tion of  speed  in  passing  from  full-load  to  no-load  (at  constant-steam  pressure 
at  the  throttle)  to  the  full-load  speed. 

61.  In  a  turbine  or  other  water-motor,  the  regulation  is  the  ratio  of  the 
maximum  variation  of  speed  from  full-load  to  no-ioad  (at  constant  head  of 
water ;    i.e.,  at   constant   difference  of   level  between  tail    race   and   head 
race),  to  the  full-load  speed. 

Variation  and  Pulsation. 

62.  In  prime  movers  which  do  not  give  an  absolutely  uniform  rate  of 
rotation  or  speed,  as  in  steam  engines,  the  "  variation  "  is  the  maximum 
angular  displacement  in  position  of    the  revolving  member  expressed  in 
degrees,  from  the  position  it  wrould  occupy  with  uniform  rotation,  and  with 
one  revolution  as  360°;  and  the  pulsation  is  the  ratio  of  the  maximum 
change  of  speed  in  an  engine  cycle  to  the  average  speed. 

63.  In  alternators  or  alterating-current  circuits  in  general,  the  variation 
is  the  maximum  difference  in  phase  of  the  generated  wave  of  E.M.F.  from 
a  wave  of  absolutely  constant  frequency,  expressed  in  degrees,  and  is  due 
to  the   variation  of  the  prime-mover.     The  pulsation  is   the  ratio  of  the 
maximum    change   of   frequency  during   an  engine  cycle  to  the  average 
frequency. 

64.  If  11  zr  number  of  poles,  the  variation  of  an  alternator  is  —times  the 
variation  of  its  prime-mover  if  direct-connected,  and  -p  times  the  variation 

of  the  prime-mover  if  rigidly  connected  thereto  in  the  velocity  ration^. 

65.  The  pulsation  of  an  alternating  current  circuit  is  the  same  as  the 
pulsation  of  the  prime-mover  of  its  alternator. 


RATING. 

66.  Both  electrical  and  mechanical  power  should  be  expressed  in  kilo- 
watts,   except  when    otherwise    specified.       Alternating-current    apparatus 
should  be  rated  in  kilowatts  on  the  basis  of  non-inductive  condition ;  i.e., 
with  the  current  in  phase  with  the  tenninal  voltage. 

67.  Thus  the  electric  power  generated  by  an  alternating-current  appara- 
tus equals  its  rating  only  at  non-inductive  load,  that  is  when  the  current  is 
in  phase  with  the  terminal  voltage. 

68.  Apparent  power  should  be  expressed  in  kilovolt-amperes  as  distin- 
guished from  real  power  in  kilowatts. 

69.  If  a  power-factor  other  than  100%  is  specified,  the  rating  should  be 
expressed  in  kilovolt-amperes  and  power-factor,  at  full-load. 


2?8       POLYPHASE   APPARATUS    AND    SYSTEMS. 

70.  The  full-load  current  of  an  electric  generator  is  that  current  which 
with  the  rated  full-load  terminal  voltage  gives  the  rated   kilowatts,  but  in 
alternating-current  apparatus  only  at  non-inductive  load. 

71.  Thus  in  machines  in  which  the  full-load  voltage  differs  from  the  no- 
load  voltage,  the  full-load  current  should  refer  to  the  former. 

If  P  —  rating   of  an  electric  generator  and  E  —  full-load  terminal  volt- 
age, the  full-load  current  is  : 

/  —  _  in  a  continuous-current  machine  or  single  phase  alternator. 
1^, 

p 
I  —      _  _  in  a  three-phase  alternator. 


I  =  _  in  a  quarter-phase  alternator. 

72.  Constant  -current  machines,  such  as  series  arc-light  generators,  should 
be  rated  in  kilowatts  based  on  terminal  volts  and  amperes  at  full-load. 

73.  The  rating  of  a  fuse  or  circuit  breaker  should  be  the  current  strength 
at  which  it  will  open  the  circuit,  and  not  the  working-current  strength. 

Classification  of  Voltages  and  Frequencies. 

74.  In    direct-current,    low-tension    generators,    the     following    average 
terminal  voltages  are  in  general  use  and  are  recommended  : 

125  volts.  250  volts.  550  volts. 

75.  In    direct  -current,    and    alternating-current,     low-pressure     circuits, 
the    following    average    terminal    voltages    are    in    general     use    and    are 
recommended  : 

1  10  volts.  220  volts. 

In  direct  current  power  circuits,  for  railway  and  other  service,  500  volts 
may  be  considered  as  standard. 

76.  In  alternating-current,  high-pressure  circuits  at  the  receiving  end,  the 
following  pressures  are  in  general  use,  and  are  recommended  : 

1000  volts.  2000  volts.  3000  volts.  6000  volts. 

10000  volts.  15000  volts.  20000  volts. 

77.  In  alternating-current    high-pressure    generators,  or  generating  sys- 
tems, the  following  terminal   voltages  are  in  general  use  and  are  recom- 
mended : 

1  150  volts.  2300  volts.  345°  volts. 

These  pressures  allow  of  a  maximum  drop  in  transmission  of  15%  of  the 
pressure  at  the  receiving  end.  If  the  drop  required  is  greater  than  15%, 
the  genetator  should  be  considered  as  special. 


APPENDIX.  279 

78.  In  alternating-current  circuits,  the  following  approximate  frequencies 
are  recommended  as  desirable : 

25—-'  or  30  ^/  40  ^  60  /-w  1 20  /-»*  * 

These  frequencies  are  already  in  extensive  use,  and  it  is  deemed  advisable 
to  adhere  to  them  as  closely  as  possible. 

Overload  Capacities. 

79.  All  guarantees  on  heating,  regulation,  sparking,  etc.,  should  apply  to 
the  rated  load,  except  where   expressly  specified  otherwise,  and  in  alter- 
nating-current apparatus  to  the  current  in  phase  with  the  terminal  E.M.F. 
except  where  a  phase  displacement  is  inherent  in  the  apparatus. 

80.  All  apparatus  should  be  able  to  carry  a  reasonable  overload  without 
self-destruction  by  heating,  sparking,  mechanical  weakness,  etc.,  and  with 
an  increase  of   temperature    elevation  not  exceeding   15°  C.  above  those 
specified  for  full  loads.      See  Sees.  25  to  31. 

81.  Overload  guarantees  should  refer  to  normal  conditions  of  operation 
regarding  speed,  frequency,  voltage,  etc.,  and  to  non-inductive  conditions 
in  alternating  apparatus,  except  where  a  phase  displacement  is  inherent  in 
the  apparatus. 

82.  The  following  overload  capacities  are  recommended : 

ist.  In  direct-current  generators  and  alternating-current  generators;  25% 
for  one-half  hour. 

2nd.  In  direct -current  motors  and  synchronous  motors.  25%  for  one- 
half  hour,  95%  for  one  minute  ;  except  in  railway  motors  and  other  appa- 
ratus intended  for  intermittent  service. 

3d.    Induction  motors.     25%  for  one-half  hour,  50%  for  one  minute. 

4th.    Synchronous  converters.     50%  for  one-half  hour. 

5th.  Transformers.  25%  for  one-half  hour.  Except  in  transformers  con- 
nected to  apparatus  for  which  a  different  overload  is  guaranteed,  in  which 
case  the  same  guarantees  shall  apply  for  the  transformers  as  for  the  appa- 
ratus connected  thereto. 

6th  Exciters  of  alternators  and  other  synchronous  machines,  10%  more 
overload  than  is  required  for  the  excitation  of  the  synchronous  machine  at 
its  guaranteed  overload,  and  for  the  same  period  of  time. 

*  The  frequency  of  120—'  may  be  considered  as  covering  the  already  existing  commercial 
frequencies  between  120^  and  140-^,  and  the  frequency  of  60-^  as  covering  the  already 
existing  commercial  frequencies  between  6o/~/  and  70^. 


280       POLYPHASE    APPARATUS    AND    SYSTEMS. 

APPENDIX    I. 
EFFICIENCY. 

Efficiency  of  Phase-Displacing  Apparatus. 

In  apparatus  producing  phase  displacement  as,  for  example,  synchronous 
compensators,  exciters  of  induction  generators,  reactive  coils,  condensers, 
polarization  cells,  etc.,  the  efficiency  should  be  understood  to  be  the  ratio 
of  the  volt-ampere  activity  to  the  volt-ampere  activity  plus  power  loss. 

The  efficiency  may  be  calculated  by  determining  the  losses  individually, 
adding  to  them  the  volt-ampere  activity,  and  then  dividing  the  volt-ampere 
activity  by  the  sum. 

I st.  In  synchronous  compensators  and  exciters  of  induction  generators, 
the  determination  of  losses  is  the  same  as  in  other  synchronous  machines 
under  Sections  10  and  IT. 

end.  In  reactive  coils  the  losses  are  molecular  friction,  eddy  losses,  and 
I-r  loss.  They  should  be  measured  by  wattmeter.  The  efficiency  of  reac- 
tive coils  should  be  determined  with  a  sine  wave  of  impressed  E.Jf.F., 
except  where  expressly  specified  otherwise. 

3d.  In  condensers,  the  losses  are  due  to  dielectric  hysteresis  and  leakage, 
and  should  be  determined  by  wattmeter  with  a  sine  wave  of  E.M.F. 

4th.  In  polarization  cells,  the  losses  are  those  due  to  electric  resistivity 
and  a  loss  in  the  electrolyte  of  the  nature  of  chemical  hysteresis,  and  are 
usually  very  considerable.  They  depend  upon  the  frequency  voltage  and 
temperature,  and  should  be  determined  with  a  sine  wave  of  impressed 
E.Af.F.,  except  where  expressly  specified  otherwise. 


APPENDIX    II. 
Apparent  Efficiency. 

In  apparatus  in  which  a  phase  displacement  is  inherent  to  their  operation, 
apparent  efficiency  should  be  understood  as  the  ratio  of  net  power  output 
to  volt-ampere  input. 

Such  apparatus  comprise  induction  motors,  reactive  synchronous  con- 
verters, synchronous  converters  controlling  the  voltage  of  an  alternating 
current  system,  self-exciting  synchronous  motors,  potential  regulators,  and 
open  magnetic  circuit  transformers,  etc. 

Since  the  apparent  efficiency  of  apparatus  generating  electric  power  de- 
pends upon  the  power-factor  of  the  load,  the  apparent  efficiency,  unless 
otherwise  specified,  should  be  referred  to  a  load  power-factor  of  unity. 


APPENDIX.  28l 


APPENDIX   III. 

Power  Factor  and  Inductance  Factor 

The  power-factor  in  alternating  circuits  or  apparatus  may  be  defined  as 
the  ratio  of  the  electric  power,  in  watts,  to  volt-amperes. 

The  inductance  factor  is  to  be  considered  as  the  ratio  of  wattless  volt- 
amperes  to  total  volt-amperes. 

Thus,  if  /  .=  power-factor,  q  =  inductance  factor,  then 

^  +  ?«  =  !. 

The  power-factor  is  the 

(energy  component  of  current  or  E.M.F.}        true  power 
(total  current  or  E.M.F!)  volt  amperes 

and  the  inductance  factor  is  the 

(wattless  component  of  current  or  E.M.F.} 
(total  current  or  E.M.F.) 

Since  the  power-factor  of  apparatus  supplying  electric  power  depends 
upon  the  power-factor  of  the  load,  the  power-factor  of  the  load  should  be 
considered  as  unity,  unless  otherwise  specified. 


APPENDIX    IV. 

The  following  notation  is  recommended  :  — 

E,  e,  voltage,  E.M.F.,  potential  difference ; 

7,  2,  current ; 

P,  power ; 

4>,  magnetic  flux; 

(B,  magnetic  density; 

A',  r,  resistance ; 

X,  jc,  reactance  ; 

Z,  z,  impedance  ; 

L,  /,  inductance  ; 

C,  c,  capacity. 

Vector  quantities,  wThen  used,  should  be  denoted  by  capital  italics. 


282        POLYPHASE   APPARATUS   AND   SYSTEMS. 


APPENDIX   V. 


Table  of  Sparking  Distances  in  Air  between  Opposed  Sharp  Needle- 
Points,  for  Various  Effective  Sinusoidal  Voltages,  in  inches  and  in  centi- 
metres. 


-    BH 

., 

DISTANCE. 

'{•    O 

2   ,_   z  s;                         DISTANCE. 

>  o   a  •<    • 

3    s  ~. 

£  eg"      ^ 

INCHES. 

CMS. 

INCHES. 

CMS. 

5 

0.225 

0-57 

60                     4.65 

11.8 

10 

0.47 

1.19 

70 

5.85 

149 

15 

0.725 

1.84 

So                7.1 

18.0 

20 

I.O 

2-54 

90                8.35 

21.2 

25 

1.3 

3-3 

IOO 

9.6 

24.4 

30 

1.625 

4.1 

no              10.75 

27-3 

35 

2.O 

5.1 

1  20 

11.85 

30.1 

40 

2-45 

6.2 

130 

12.95 

32.9 

45 

2-95 

7-5 

140 

13-95 

35-4 

5° 

3-55                  9-° 

150              15.0 

3«.' 

INDEX. 


AIR  blast  transformers,  135. 
Alternating  circuit,  flow  of   cur- 
rent in,  3. 
energy  in,  10. 
Alternations,  16. 
Alternators  (see  Generators). 
Ampere  turns,  rotary  converter, 

118. 

Angle  of  lag,  11. 
Apparent  efficiency,  85. 
energy,  n. 
resistance,  14. 

Arc  lamps,  on  low  frequency  cir- 
cuits, 226. 

Armature,  inductance,  34. 
induction  motors,  63. 
multitooth  construction,  33. 
reaction,  33,  34. 
resistance   of   induction   mo- 
tors, 64. 

unitooth  construction,  34. 
Auto  converters  for   starting  in- 
duction motors,  66. 

BALANCED   three-phase  system, 

202. 
two-phase    system,    185-188, 

190. 
Blowers  for  cooling  transformers, 

1.37- 

Breakdown  point  induction  mo- 
tors, 76-78. 
synchronous  motors,  95. 


CALCULATION    of    transmission 
lines,  constants  for,  238,  241. 
Capacity,  4. 

and    magnetic  reactance    in 

same  circuit,  8. 
of  transmission  lines,  241. 
Charging  current  in  transmission 

lines,  241,  242. 

Choking  coils  for  lightning  ar- 
resters, 164. 

Coefficient  of  self-induction,  4. 
Combinations  of  circuits  in  poly- 
phase systems,  180. 
Compensators  for  induction  mo- 
tors, 66,  67. 

synchronous  motors,  99. 
Mershon,  175. 
Composite  winding  of  generators, 

38- 

Compounding  of  generators,  37. 
Condensance,  8. 
Condenser,  use  of,  with  induction 

motors,  88. 
Conductors     (see     Transmission 

lines). 

Connections  of  polyphase  wind- 
ings, 180,  194. 
delta,  181,  195,  196. 
interlinked,  181. 
ring,  182. 
star,  182. 
Y,  181,  195,  196. 
Constants  for  line  calculation,  238. 


283 


284 


INDEX. 


Control    of    alternating    current   ; 

apparatus,  145. 

Converter  (see  Rotary  converter). 
Cooling   of    transformers  by  air 

blast,  135. 
natural  draft,  139. 
oil,  127. 
water,  131. 

Copper,  amount  of,  required  with 
different  polyphase  sys- 
tems, 231. 

lossesin  transformers, 141, 142. 
Core  losses  in  transformers,  141. 
Cosine  of  lag  angle,  u. 
Counter  E.M.F.,  3. 
Currents,   alternating,    definition 

of  terms,  i. 

Current,  armature  in  rotary  con- 
verter. 117. 

in  synchronous  motor,  103. 
lagging.  4. 
leading.  5. 
Wattless,  12. 
Curve  of  E.M.F.,  2. 

generator  efficiency,  44. 
induction    motor    efficiency, 

75,  88. 

transformer  efficiency,  140. 
three-phase  E.M.F.,  194. 
two-phase  E.M.F.,  181. 
Curves,  of  line  losses,  253,  254. 
voltage  drop  in  transmission 
lines,  255. 

DELTA  connection  of  windings, 

182,  195,  196. 
Distribution  circuits,  monocyclic,    i 

215. 
three-phase     four- wire,    200, 

201,  203. 

three-phase  three-wire,  203. 
two-phase  four-wire,  187. 


Distribution  circuits,   two-phase 
three-wire,  190. 

EFFICIENCY  generators,  44. 

induction  motors,  75,  85,  88, 

synchronous  motors,  94. 

transformers,  140. 
Electrical  resonance,  259. 
Electromotive  force,  2. 

impressed,  5. 

energy  component  of,  6. 

induction  component  of,  6. 
Electromotive  force,  curve  of,  2, 

three-phase,  194. 

two-phase,  181. 
Energy  apparent,  10. 

current,  12. 

loss  in  circuit,  13. 
Engine,  regulation  for  parallel  op- 
eration of  generators,  50,  5 1 . 
Excitation,  rotary  converters,  1 16, 
119. 

synchronous  motor,  102. 
Exciting  current-of  transformers, 

142. 

Exciter  panel,  147,  149. 
Exciters,  capacities  of,  for  gener- 
ators and  motors,  104. 

FACTOR,  induction,  n. 

power,  1 1. 
Farad,  the,  5. 
Field  induction  motor,  71. 
Field  excitation,  generator,  37. 

rotary  converter.  117-119. 

rotary   converter,    effect    on 
voltage,  119. 

synchronous  motor,  103. 
Flux,  magnetic,  3. 
Frequency  changer,  176. 
Frequency,  choice  of,  224,  228. 

definition,  16. 


I-NDEX. 


285 


Frequency,  effect  of,  on  parallel 
operation  of  generator,  228. 
high,  224. 

induction  motors,  83. 
limit    of    rotary    converters, 

118. 

low,  209. 
Fuses,  159. 

GENERATOR,  armature  construc- 
tion, 20,  33. 

armature  inductance,  34. 

armature  reaction,  34. 

armature  windings,  32. 
Generators : 

conditions  effecting  cost,  60. 

efficiency,  44. 

electro-motive  force,  34. 

elementary  forms,  17. 

field  excitation,  34. 

inductor  type,  27. 

losses,  44. 

methods  of  driving,  52. 

monocyclic  windings,  213. 

parallel  running,  48,  51. 

revolving  armature  type,  18. 

revolving  field  type,  21. 

single-phase  output,  210. 

speed,  47. 

speed  regulation  of  engine  for 
driving,  50. 

three-phase,  210. 

three-phase  windings,  195. 

two-phase  windings,  180-182. 
Geographical  illustrations  of  line 
losses,  253,  254. 

voltage  drops,  255. 
Grounding  of  lightning  arresters, 
1 66. 

HARMONIC  motion,  simple,  2. 
Henry,  the,  4. 


High  voltage  generators,  32. 
High-tension  stitches,  152. 

IDLE  currents  (see  Wattless  cur- 
rent). 

Impedance,  7. 
Impressed  E.M.F.,  5. 
Inductance,  3. 
Induction,  4. 

compound  E.M.F.,  6. 

factor,  1 1. 
Induction  motors,  63. 

condensers  for,  88. 

construction  of  primary  and 
secondary,  69. 

efficiency,  85. 

frequency,  83. 

initial  voltages,  85. 

methods  of  starting,  64. 

monocyclic,  221. 

principles  of  operation,  63. 

power  factor,  85. 

single-phase,  89. 

speed  regulation,  78. 

starting   torque  and  current, 

73- 
transformer,    capacities    for, 

86. 
variable  armature  resistance 

type,  64,  72,  78. 
voltage,  84. 
wiring  for,  85. 

with     short-circuited     arma- 
tures, 72,  78. 
Inductive  loads,  85,  102. 
Inductor  generator,  27. 
Insulators,  167. 
glass,  167-169. 
porcelain,  167. 
provo  type,  169. 
Iron  losses,  generators,  46. 
transformers,  142. 


286 


INDEX. 


LAG,  angle  of,  u. 
Lightning  arresters,  160. 
G.  E.  type,  162. 
installation  of,  163. 
Lightning    arresters,    protection, 

1 60. 

Wurtz  type,  162. 
Line  (see  Transmission  lines). 
Line  constants  for  power  trans- 
mission, 241. 

protection  from  lightning  ef- 
fects, 1 60. 
Lines  of  force,  3. 
Load,  maximum  induction  motor, 

78,  88. 

sychronous  motor,  93. 
Long  distance    power   transmis- 
sion by  three-phase  system, 
205. 

by  two-phase  system,  187. 
Losses  in  generators,  44. 
induction  motors,  87. 
transformers,  140. 

MAGNETIC  circuit,  inductor  gen- 
erator, 29. 

revolving  field  generator,  23. 
Magnetic  field  induction  motor, 

63- 

Magnetizing  current,  76. 
Measurement  of  power  in  mono- 
cyclic  circuits,  223. 
three-phase  circuits,  201. 
two-phase  circuits,  190. 
Mesh  connection  (see  Ring  con- 
nection). 
Monocyclic  system,  212. 

distributing  circuits,  215. 
features  of,  212. 
generator    armature  connec- 
tions, 213. 
measurement  of  power  in, 223. 


Monocyclic  system,  motors  for, 

221. 

transformation       to       three- 
phase,  217. 
transformer  connections  for 

motors  and  lights,  217. 
Motor  connections  in  three-phase 

system,  199. 

two-phase  system,  184,  191. 
Motor  generators,  178. 
Multiphase  (see  Polyphase). 

NEUTRAL    point  in   three-phase 
system,  194,  196,  201. 

OHM'S  law,  modification  of,  in  al- 
ternating current  circuits,  3. 

Oil  switches,  152. 

Oiled  cooled  transformers,  127. 

Oscillatory  character  of  lightning 
discharges,  164. 

Oscillatory     character    of    inter- 
rupted circuits,  259. 

Output   maximum    of    induction 

motors,  78,  88. 
synchronous  motors,  95. 

PARALLEL    running  of    genera- 
tors, 48. 

Periodicity  (see  Frequency). 

Phase   displacement   (see  Angle 
of  lag). 

Phase  transformation,  185. 

Polyphase  circuits,  various  con- 
nections of  (see  Two-phase, 
Three-phase,    and     Mono- 
cyclic  systems), 
currents,  180. 
systems    and    combinations, 

1 80. 
transformers,  125. 

Power  factor,  n. 


INDEX. 


287 


Power  factor,  induction  motors, 

85. 

rotary  converters,  121. 
synchronous  motors,  105. 
Power  measurement,  monocyclic 

system,  223. 

three-phase  system,  201. 
two-phase  system,  190. 
Power  transmission,  long  distance 
by  three-phase  system,.2O5. 
two-phase  system,  187. 
Primary  of  induction  motor,  63, 

69. 

Prime  movers  for  driving  genera- 
tors, 52. 

Pressure  Regulators,  171. 
automatic,  174. 
polyphase  type,  172. 
single-phase  type,  171. 
Stillwell  type,  172. 
Punchings,   generator  armature, 
32. 

RADIATING  surface  of  transform- 
ers, 126. 
Ratio  of  transformation  of  rotary 

converters,  113. 
transformers,  144. 
Reactance,  7. 

of  transmission   conductors, 

241. 
Reaction,    generator    armatures, 

33>  34- 

Rectifiers,  175. 

Regulation,  inherent  of  genera- 
tors, 43. 

of  transformers,  143. 
speed,  of  induction   motors, 

78. 

of  synchronous  motors,  94,  95 . 
Regulators  (see  Pressure  regula- 
tors). 


Resistance  apparent,  14. 

copper  conductors,  241. 

virtual,  9. 

Resonance  effect,  259. 
Reversing  induction  motors,  69. 
Ring  winding,  182,  195. 
Rotary  converters,  109. 

armature  connections,  no. 

armature  reaction,  118. 

general  features,  109. 

limit  of  frequency,  118. 
Rotary  converters  made  from  di- 
rect current  generators,  109. 

parallel  operation,  124. 

power  factor,  121. 

ratio  alternating  to  direct  cur- 
rent voltage,  113. 

six-phase,  115. 

starting  and  running,  123. 

types  and  uses,  116. 

voltage  variation,  119,  120. 
Rotor  of  induction  motor,  63,  70. 

SECONDARY  systems  of  distribu- 
tion, monocyclic,  215. 
three-phase,  four-wire,  199. 
three-phase    three-wire,    195, 

203. 

two-phase  four-wire,  187. 
two-phase  three-wire,  190. 
Self-induction,  coefficient  of,  4. 
Simple  harmonic  motion,  2. 
Sine  wave,  2. 
Single-phase    induction    motors, 

89. 

synchronous  motors,  95. 
Single-phase  output   three-phase 

generators,  210. 
Skin  effect,  10. 
Slip  of  induction  motors,  64. 
Speed  control   of  induction  mo- 
tors, 79. 


288 


INDEX. 


Speed,  effect  of,  on  cost  of  gen- 
erators, 60. 

regulation  of  engines  for  par- 
allel running,  50. 
variation    of    induction    mo- 
tors, 78. 
Star  connection  of  windings,  182, 

195- 

Starting   current    induction    mo- 
tors, 64,  73. 

synchronous  motors,  97. 
Starting  of  induction  motors,  64. 

rotary  converters,  123. 

synchronous  motors,  97. 
Starting  torque  effect  of  voltage 
on  induction  motors,  74.  77. 

on  synchronous  motors,  96. 
Static      transformers      (see 

Transformers). 
Switchboards,  146,  207. 
Switches.  152. 

air  break,  153,  154. 

expulsion,  154. 

oil,  154. 

Synchronizing  devices.  167. 
Synchronous  motors,  94. 

advantages  of,  94. 

field  excitation,  102. 

instability,  108. 

methods  of  starting,  97. 

power  factor,  105. 

speed.  95. 

torque  and  output,  95. 

used  as  condensers,  104. 

voltage,  96. 

TEMPERATURE   of  transformers, 

127. 
Theory   of    action   of    induction 

motors,  63. 
Three-phase    circuits   for   power 

distribution,  203. 


Three-phase  circuits  for  lighting 
distribution,  209. 

railway  distribution,  207. 

curves  of  E.M.F.  194. 

four-wire  system,  197. 

long  distance  transmission 
circuits,  205. 

motor  connections,  199. 

three- wire  system,  195. 

transformer  connections,  195. 
Three-phase  system,  194. 

measurement  of  power  in.  201. 
Torque  diagram  of  induction 
motors,  74. 

starting  of  induction  motors, 

73- 

starting  of  synchronous  mo- 
tors, 95. 

Transformation  of  phases,  185. 
Transformer  connections,  mono- 
cyclic,  217. 

six-phase,  198. 

three-phase,  195. 

two-phase,  184. 
Transformers,  125. 

air  blast  type,  135. 

efficiency,  140. 

losses,  140. 

natural  draft  type,  139. 
Transformers,    operation    of    air 
blast,  137. 

polyphase,  125. 

regulation,  143. 

self-cooled  oil  type,  127. 

water-cooled  oil  type,  131. 
Transmission     lines,    calculation 
of.  238. 

capacity  of,  241. 

charging  current  in,  241. 

inductance  of,  241. 

resistance  of,  241. 

voltage  drop  in,  238,  243,  255. 


INDEX. 


289 


Two-phase  four-wire  system,  187. 

generator    armature   connec- 
tions, 181. 

interlinked  windings,  181. 

separate  windings,  181. 

three-wire  system,  190. 

to  three-phase,  185. 

transformer  connections,  184. 

unbalancing,  191. 
Two-phase  system,  180. 

relations  of  E.M.F.  in,  180. 

UNIT  of  capacity,  5. 

of  self-inductance,  4. 

VOLTAGE   drop   in    transmission 
lines,  238,  243,  255. 

Voltage,  effects  of,  on  output  of 
induction  motor,  84. 

Voltage   of    synchronous   motor, 
96. 

Voltage  of  induction  motor,  84. 
relation   of    line   to   induced 
E.M.F.  in  three-phase  gen- 
erators, 36. 


WATER  wheels  as  prime  movers, 

49,  52>  56- 

Wattless  current,  12. 
Wattless  or  magnetizing  current 
in  induction  motor,  88. 

transformers,  142. 
Wattmeter  for  measuring  power, 
in  monocyclic  circuits,  223. 

three-phase  circuits,  200,  202. 

two-phase  circuits,  190. 
Watts  apparent,  n. 
Windings,  generator  armature,  32. 

interlinked,  181. 

monocyclic,  213. 

three-phase,  195. 

two-phase,  18 1. 

primary  of  induction  motor, 

7i. 

secondary  of   induction   mo- 
tor, 72. 
Wiring  formulas,  244. 

application  of,  247. 

Y  connection  in  three-phase  sys- 
tem, 195,  196. 


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